It really depends on the design of the rims especially the spokes. I have +33 at the front and it didn't clear the calipers. Had to put in a real thin spacer. 2-3mm to clear them. The rears are usually not a problem.
when the is -vacuum it will suck the diaframe to lower down the pressure, full throttle when 0 vacuum most of the stock regulator will rise to 41psi.
how about boost?
will the air pressure push the diaframe down and...