[Quicksilver]

Valve train rev limits aren't a Porsche specific issue. A valve, retainer, and spring is a really generic system. You have a mass of the pieces, and a size limitation for the spring package.
Another variable is that higher RPM will require a different cam and those will be designed with more "ramp" that can deal with controlling the valve acceleration for higher RPMs.

The big Porsche limitation will be that rebuilds are going to be more expensive than other engines and that most people expect a Porsche engine to have a very long life that goes far beyond the lifespan of other high performance motors.