1988 Roller Cam 318 'entry-level' power potential?

More HP? It all depends on where the torque peak moved to. I'd bet the torque peak moved lower with the roller cam (a good thing for the loooow rear gear) , and so peak HP may not have increased; that is really inconsequential though. If your torque increased through a significant part of the lower and mid RPM range, then the 'scoot' of the car HAS increased, regardless of where the peak HP number ended up at.

BTW, I like your car!