30A breakers don't pop immediately. (
http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf) So, a 30A breaker pulling 80A (which according to tests I've heard about is about the max you can get to a CIM given other loads on the system and the battery) will last 0.8 - 1.8 seconds. A 40A breaker will give you about twice that time. But that's assuming you're practically stalling the CIM. And it completely ignores the fact that, more than likely, the RoboRio will cut your motor power due to voltage drop.
Acceleration is also a function of code/driving/operator interface. All too often I see swerves driven like tanks. Go to point, pivot wheels, go sideways, pivot wheels again, go back to going forward. If you watch how teams who have mastered swerve (ok, I'm a 16 fan boy, I'll admit it) they tend to be much more fluid and rely less on raw power to accelerate from 0 in that direction. 254 does similar things when it comes to tank drives, it's why they seem to get away with gearing so much higher than other people.
Basically, while being able to pour more power for longer may seem like a good solution to the acceleration problem there's a fair bit more to it and utilizing it will mean your motors run cooler, your batteries last longer, and you can STILL out swerve most people.
That all being said, if I could easily find a 40A rated slip ring for comparable weight/cost of the 30A I'd choose it every time. But if I given the choice between a more complicated module (bevel), more complicated code (limit rotations), and limited continuous current (30A breaker) I'd choose limited current every time. But I'm a software engineer, I have a certain set of criteria I apply to decisions. Other folks with different backgrounds might make different decisions.
