If the build of the motor only makes the power at high rpms, thats where you will be using it.
Larger valves are for higher rpms as are higher lift cams.
If you are happy with 350-375whp then the stock valve sizes will work and give better low rpm torque, I'm not sure exactally where that hp number falls, depending on the boost.
Most stock size valve motors even with better cams giveup the power at below 7k.
Multiple angle valve jobs always help flow, usually the low lift area shows the most improvement, at least that's what I have read. As someone already posted the valve spends twice as much time in low lift as high lift.
We flowed some ported heads before and after a performance valve job and really didn't see much improvement but will still go for a multiangle job every time.
I would buy the best valves I can afford. As in: iconel, backcut, and coated, these things all help the valves live longer in a turbo application.