I'm still not sure how you're coming to the conclusion that PSP version is more powerful at lower levels. Reminder that the +100 is not multiplied by Ultima's level. It's only relevant for very low level sums.
Try using the entire damage formulae, including critical hits and random boni. Assuming I've got enough mental energy left to do this correctly, the average damage from the PSP version based on
https://finalfantasy.fandom.com/wiki/Ultima_(Final_Fantasy_II) should be:
<Average Damage> = (<Ultima Level> * (<Tier> + floor(<Tier> / 2)) ^ 2 + 100) * 1.05 + 0.5 * ((<Ultima Level> * (<Tier> + floor(<Tier> / 2)) ^ 2 + 100) % 256)
giving (rounded to the nearest integer):
Ultima Level / Tier | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
1 | 155 | 157 | 169 | 180 | 211 | 231 | 281 | 310 | 378 | 289 | 376 | 424 | 529 | 587 | 583 | 649 | 792 |
2 | 155 | 158 | 183 | 205 | 267 | 307 | 278 | 337 | 473 | 551 | 597 | 693 | 903 | 890 | 1138 | 1143 | 1429 |
3 | 155 | 160 | 197 | 229 | 322 | 383 | 404 | 492 | 569 | 685 | 817 | 961 | 1150 | 1322 | 1566 | 1638 | 1937 |
4 | 155 | 161 | 211 | 254 | 378 | 331 | 529 | 647 | 792 | 819 | 1166 | 1230 | 1524 | 1625 | 1993 | 2260 | 2574 |
5 | 155 | 163 | 225 | 279 | 306 | 407 | 655 | 674 | 887 | 1081 | 1387 | 1499 | 1898 | 2057 | 2421 | 2754 | 3211 |
6 | 155 | 164 | 239 | 304 | 362 | 483 | 652 | 829 | 1110 | 1215 | 1608 | 1768 | 2272 | 2488 | 2976 | 3248 | 3848 |
7 | 155 | 166 | 253 | 329 | 418 | 559 | 778 | 856 | 1205 | 1349 | 1828 | 2037 | 2518 | 2792 | 3404 | 3742 | 4357 |
8 | 155 | 167 | 267 | 353 | 473 | 635 | 903 | 1011 | 1429 | 1611 | 2049 | 2305 | 2893 | 3223 | 3831 | 4237 | 4993 |
9 | 155 | 169 | 281 | 378 | 529 | 583 | 901 | 1166 | 1524 | 1745 | 2270 | 2574 | 3267 | 3527 | 4387 | 4731 | 5630 |
10 | 155 | 171 | 295 | 275 | 585 | 659 | 1027 | 1193 | 1619 | 2007 | 2491 | 2843 | 3513 | 3959 | 4815 | 5225 | 6267 |
11 | 155 | 172 | 308 | 300 | 641 | 734 | 1152 | 1348 | 1842 | 2140 | 2711 | 3112 | 3887 | 4390 | 5242 | 5719 | 6776 |
12 | 155 | 174 | 322 | 325 | 569 | 810 | 1150 | 1375 | 1937 | 2274 | 3060 | 3381 | 4261 | 4694 | 5670 | 6213 | 7413 |
13 | 155 | 175 | 336 | 349 | 624 | 886 | 1275 | 1530 | 2161 | 2536 | 3281 | 3649 | 4636 | 5125 | 6225 | 6836 | 8049 |
14 | 155 | 177 | 350 | 374 | 680 | 834 | 1401 | 1685 | 2256 | 2670 | 3502 | 3918 | 4882 | 5429 | 6653 | 7330 | 8686 |
15 | 155 | 178 | 364 | 399 | 736 | 910 | 1398 | 1712 | 2479 | 2804 | 3722 | 4187 | 5256 | 5860 | 7080 | 7824 | 9195 |
16 | 155 | 180 | 378 | 424 | 792 | 986 | 1524 | 1867 | 2574 | 3066 | 3943 | 4456 | 5630 | 6292 | 7636 | 8318 | 9832 |
Based on those numbers, however, I'm questioning whether the linked formula is correct, as the damage growth from one tier to the next is pretty erratic and there are several places where gaining a tier actually reduces the average damage (e.g. at tier 9, Ultima 1 does about 90 damage less than at tier 8 since tier 9's maximum random damage = ((9+4)^2+100)%256 = 13 vs. tier 8's ((8+4)^2+100)%256 = 244).
Our version takes an extra variable (caster's Spirit), so to try to keep the comparison somewhat even, let's define a tier for our version as a percentage of the caster's combined skill + stat power out of a total of 16, so e.g. a caster with a total of 24 * 16 = 384 skill points and 99 Spirit has a tier of 16, a caster with 192 skill points and 50 Spirit has a tier of 8, etc. It's not entirely a fair comparison since Spirit is both easier to raise and has a larger marginal effect on average damage than do skill levels, but it's good enough for hand-waving. In that case, the average damage from our version should be:
<Average Damage> = (<Ultima Level> * (1.5 + min(0.5, <Tier> / 16))) * (25 + floor(99 * <Tier> / 16 / 4) + floor(24 * (<Tier> - 1) / 8)) * 1.55
giving (rounded to the nearest integer):
Ultima Level / Tier | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
1 | 58 | 63 | 78 | 92 | 109 | 124 | 142 | 159 | 180 | 192 | 208 | 223 | 236 | 251 | 264 | 279 | 291 |
2 | 116 | 126 | 156 | 183 | 217 | 247 | 285 | 318 | 360 | 384 | 415 | 446 | 471 | 502 | 527 | 558 | 583 |
3 | 174 | 189 | 234 | 275 | 326 | 371 | 427 | 477 | 539 | 577 | 623 | 670 | 707 | 753 | 791 | 837 | 874 |
4 | 233 | 252 | 312 | 366 | 434 | 494 | 570 | 637 | 719 | 769 | 831 | 893 | 942 | 1004 | 1054 | 1116 | 1166 |
5 | 291 | 315 | 390 | 458 | 543 | 618 | 712 | 796 | 899 | 961 | 1039 | 1116 | 1178 | 1256 | 1318 | 1395 | 1457 |
6 | 349 | 378 | 468 | 549 | 651 | 742 | 854 | 955 | 1079 | 1153 | 1246 | 1339 | 1414 | 1507 | 1581 | 1674 | 1748 |
7 | 407 | 441 | 547 | 641 | 760 | 865 | 997 | 1114 | 1259 | 1345 | 1454 | 1562 | 1649 | 1758 | 1845 | 1953 | 2040 |
8 | 465 | 504 | 625 | 732 | 868 | 989 | 1139 | 1273 | 1438 | 1538 | 1662 | 1786 | 1885 | 2009 | 2108 | 2232 | 2331 |
9 | 523 | 567 | 703 | 824 | 977 | 1113 | 1282 | 1432 | 1618 | 1730 | 1869 | 2009 | 2120 | 2260 | 2372 | 2511 | 2623 |
10 | 581 | 630 | 781 | 915 | 1085 | 1236 | 1424 | 1592 | 1798 | 1922 | 2077 | 2232 | 2356 | 2511 | 2635 | 2790 | 2914 |
11 | 639 | 693 | 859 | 1007 | 1194 | 1360 | 1566 | 1751 | 1978 | 2114 | 2285 | 2455 | 2592 | 2762 | 2899 | 3069 | 3205 |
12 | 698 | 756 | 937 | 1099 | 1302 | 1483 | 1709 | 1910 | 2158 | 2306 | 2492 | 2678 | 2827 | 3013 | 3162 | 3348 | 3497 |
13 | 756 | 819 | 1015 | 1190 | 1411 | 1607 | 1851 | 2069 | 2337 | 2499 | 2700 | 2902 | 3063 | 3264 | 3426 | 3627 | 3788 |
14 | 814 | 882 | 1093 | 1282 | 1519 | 1731 | 1994 | 2228 | 2517 | 2691 | 2908 | 3125 | 3298 | 3515 | 3689 | 3906 | 4080 |
15 | 872 | 945 | 1171 | 1373 | 1628 | 1854 | 2136 | 2387 | 2697 | 2883 | 3116 | 3348 | 3534 | 3767 | 3953 | 4185 | 4371 |
16 | 930 | 1008 | 1249 | 1465 | 1736 | 1978 | 2279 | 2547 | 2877 | 3075 | 3323 | 3571 | 3770 | 4018 | 4216 | 4464 | 4662 |
The damage growth here is much less erratic, plus if we drop the tier simplification and look at the base skill levels and Spirit stat, we get damage growth with every 8 skill levels (vs. 24 for PSP) and every 4 points of Spirit; furthermore, every point of Spirit increases accuracy by 1% (up to a cap of 50% since Ultima has a base accuracy of 50%, but Spirit beyond 50 is still useful for counteracting accuracy reduction due to equipment penalties), making extra hits more likely and raising the average damage.
As for expected player stats/skill levels, our version of Ultima is the same in both the Bug Fix and Restored patches, but Restored's passive skill growth makes it pretty easy to have level 7+ in all weapon skills and level 9+ in all spell skills by the endgame, plus whatever levels you get from active usage. Give Ultima to somebody with a decent Spirit stat, and they'll likely be dealing damage around the tier 10 level with Ultima 8+, at which point the PSP version deals about 400 more damage (~20%). Of course, there are only about a dozen monsters in the entire game that have high enough max HP for that 400 damage to make a difference

.