Finishing my no coins run now....I honestly can't tell the difference without mario coins. I notice more fire gates and enemies, but nothing specific. I now have 700s on 8-5 without coins (which seems to me makes it easier than with the coins), and there was a hidden powerup on 8-4 that is now gone. I may be rushing through it too fast?
Here's what I'd suggest about the game, having beating it twice:
1. the 100 coin = star thing is a terrific gameplay element...except when it's not. Getting a star right before or in the middle of a turtle jumping segment, which are pretty prevalent in the game, is frustrating. Maybe consider having the select button to cancel the star? Or, less ideal, 100 coins gives you a star in reserve you can activate with select.
2. This was probably present in the original game and can't easily be changed. When mario is up around the HUD, he can't shoot. Get's me hit a few times, and lakitus laugh at me.
3. I think you're aware of this one, but getting hit over open water is instadeath because you sink. However, it also seems way too easy to die when youre close to the bottom of a water level...like the swim button stops registering similar to not being able to shoot up top. This problem doesnt occur in the water gap in 8-5, because it doesnt have the whirlpool effect probably.
4. The first time stopper in 6-4 seems to freeze enemies in a way where you cant finish the jump
5. The music note boxes just seem frustrating and unpredictable, rather than challenging.
6. The turtle at the beginning of 7-2 likes to spaz out. not a big deal
7. It's good that you made the game beatable with no mario coins. it has a much different and equally strong gamefeel when you can move through the levels quickly
8. Ok this last one is more of a personal point, but I love hammer bros. It would be cool if once you beat it, and play through again, every enemy is a hammer bro, but youd need to put wings on many of them (or only do it for ground enemies, since hammerbro jumping would be tricky due to, ykno, the hammers)
I hope this doesnt sound overly critical. Love this hack, its one of those cases where so much is done right that there's only small fine tuning things to talk about.