would installin racing pistons race my compression? enough to blow the motor? enough to need an octane booster or something?

 

ya, no, ya

 

it depends on how far you raise it. Each bike has a limit that it can take on 92octane gas. You need to find out what that limit is before you dive in to you motor.

 

what is the compression limit on a 400ex

 

I have no idea. On my blaster I was safe to raise the compression by shaving the head. I shaved off 0.040 from it and that is the absolute limit on a blaster unless you are running higher octane.
You should talk to someone who knows 400's better then i do.