I'd personally be okay with it if it was less of a speed boost and more of a midair redirection. As much as I don't like race thokfests, I still have to admit it's useful, hell, almost essential at points, in negotiating certain tight corners. If you ask me, thokspeed should be equal to or lesser than Sonic's actual top speed just to make straight-line thokking pointless.
As far as Tails and Knux advantages goes, well, that's more a level design issue than anything else. It's hardly impossible to make a race map that balances the pros and cons of all the characters - it's just that nobody ever bothers to do so.