FRS,

As Mike has just said, the accleration in a 500+ car is ferocious. You simply are not in the 4500 - 6000rpm region long enough to suffer any ACT issues brought about by running 38psi.

I do this primarily for when changing gear as its normal to drop back to around 5500rpm after the gear change so the quick 38psi boost just gives the car a shove up the backside for a brief second!

It's rare you plant your foot in 5th from 4500rpm, with a T4 car you normally down change to 4th and use the extra revs. Nevertheless as Mike says, if you do plant your foot at 4500rpm in 5th it will be no more than about 3 seconds untill your past 6000rpm anyway!!!