I'm in the middle of replacing the head gaskets on my z, and I noticed the valves all have a slight dish to them. I'm assuming this has something to do with the car being overheated, correct? Is it a bad idea to just put everything back together this way or should I replace them? Will I see a loss in performance if I reuse them?

 

I would at least clean the carbon off them and re-lap them.

 

The dish is made there. It is to prevent reversion.
Ya need to stand the heads on the side and fill the port's with solvent.If any runs out in the chamber from the E or I, ya should have a valve job. Do the intake,then the exhaust.