The levels are okay for race... but the overly large amount of enemies coupled with one-way routes makes exploration and backtracking irritating and a needless chore. I'd cut back on the number of crawlas, jetty-syns and crawla commanders if I were you.
Some enemy variation would be nice as well, and try not to put crawlas directly on a ledge the player can only get up to via a spring. It's horribly irritating to be hit, fall back down, and then lose invincibility while you're rising again.
The special stages are still evil and unforgiving, with an insane difficulty curve. If you're going to eventually make seven of them, I'd suggest to postpone #3 to the fifth or sixth stage.
More of a technical issue, but as said, the water colormaps don't look particularly great in 2.0's palette, you should go back and try to figure out more pleasant-looking ones.
Other than that, the standard for level length sure has changed from 1.09 to 2.0, and this is pretty obvious. All the levels can be completed in under or around a minute, which is pretty short in comparison to Deep Sea, Castle Eggman, Arid Canyon, Egg Rock, etc. Why don't you pull a Mystic and combine two acts into one and make a brand new second act? ;)
Also, it's about time you made those fences in WVZ2 and GMZ2 solid. :7 And use the Greenflower remix for WVZ1.
EDIT: (Actually repost, but)
I finally got off my ass and beat this with all emeralds as Sonic, and ran into a few bugs.
When I reached Slime Silo, my save turned into a "Completed!" save. After beating Slime Silo, it changed to Egg Rock. I'm assuming this is a bug in SRB2 itself, but it's part of a worse nuisance.
According to the secrets checklist, I was supposed to have level select already. This is probably a fault on your end, because the needed grade for all emeralds changed in 2.0. This means I can't go emblem hunting without replaying the entire game.
Also, did you use ModBy? If so, then the game is bugged and doesn't show your name in the last page of the credits. All I got was the same pencil drawing of Sonic from the previous page with no text before it went to the emerald screen.
Lastly, because Eggman runs away from you, Slime Silo is more irritating than ever. Eggman keeps rising until you can't reach him, so you have to jump below him until he wises up and lowers a tad.