Oversized valves: good or bad?

Like was said.. If the only change is the size of the valves, there will be a small gain due to the fresh seat and the seat being moved out to the edge of the bowl. There will also be a very small gain in very low-lift flow. How much and for how far in terms of lift that gain helps is a function of the intake system that feeds it (which includes the head port, the intake manifold port, the plenum, and the carb) plus the rate of lift of the cam. I always go oversize because of those gains. There is generally no percieved loss by going slightly larger unless you go too large on the port, intake, carb, or cam, in which case the engine will still make more power, but the rpm range of that peak will move up to where the intake system is at peak efficiency. I say percieved loss because the bench might show it, but with the rest of the components a dyno doesnt seem to note any loss.
In terms of a boosted engine, the idea is bigger is always better. At least in my circles which are primarilly V8. Boost is only a reading of resistance to flow. You're measuring how hard it is to ram air into the engine. So a ported head at 5psi will move a bit more air than an unported head at 8psi. You'll also really need to examine the turbo you want to use if porting is being done. Because it's pretty easy to undersize the turbo's compressor side when the airflow of the intake side is increased by 15-25%.