Has anybody installed the thinner head gasket, on the 400EX, to increase compression?

 

For the amount of work it'll be...your not going to gain much if you can even find a thinner head gasket. You may get a point or two but nothing you're going to notice. If you want to take the time to pull the head, check valve clearance with putty and have the head milled down some.
I'm assuming with all the more compression the 400ex has, this should be possible.
You may want to check into another piston. There may be such an animal that uses the same bore but has a higher dome to up the compression ratio. If one does exist, deglaze the cylinder bore with a hone, install the new piston, rings, & circlips, & reassmble. Viola...more compression the right way.

 

Yes, I installed a xr400 head gasket on mine, I assume that this is the difference in compression from bike to bike, I also messed with the intake and exhaust of the engine and with a 165 main and a uni filter, I haven't been beaten yet in a drag race even with bikes with aftermarket pipes. Going to do the 426 kit and a cam either this winter or next.....