Someone has a 29 on the leaderboard, but they must do something different to me. However, I have tested everything I can think of. I thought I would post this on here to maybe get some more minds to look at it and think about the possibilities.

I would not use Vector and use someone with fast starts (ie. Silver). Other than that I would just keep trying until you get a good time.

Vector gives the bests starts. I tested characters for few hours with the fast starts. He has gotten me the fastest time to the 2nd character. Plus Vector is best on 100m, so it make sense to use him.

Now I have a 30.003 with Eggman, Shadow, Vector and Metal Sonic. Still not confident about the character combo being completely optimal, but it sure works well. Eggman has a long boost for some reason even though he has fast starts, which makes him really good for the start. (The best I have found).

Eggman, Metal Sonic, Bowser and Yoshi is the best combo. Took a couple months to figure out, but that's the optimal combo ;)