I have a gsr head completely redone, it has supertech flat faced
valves and valvesprings. Are these good to use on a FI motor? Or are these better for NA?

 

Better for NA but they will work. The flat valves tend to bump the CR up a slight bit.

 

they are working great in my car

 

With NA apps, you want as high a CR as you can get with your gas. WIth boost, CR really doesnt matter, at least its not as cruicial. I'd rather not have flat-faced valves, as they are heavier. There is a smaller chance of floating a valve if you rev it really high, although valve failure is usually from bent valves, not floating them.

Cliffs: flat valves are for increasing CR when you can't get the right pistons...