I found the correct ROM offset for Sonic's top speed in Sonic 1.
Not sure how many versions there are of this game, but the version I used to find the correct offset was "Sonic The Hedgehog (USA, Europe)" with a CRC-32 Checksum of F9394E97.
At ROM address $012C2E, the normal value is "06". Change this value to raise or lower his top speed. Note that the value is loaded only once when the game first starts.
EDIT: I tried values of 00 and 01. A value of 00 won't let him move at all, and a value of 01 makes him move pretty slow. Higher values seem to make him move faster on top end, but hard to find long areas where he can reach top speed, so it's hard for me to tell.