I know I need mine replaced but I have a question....Can this cause my Z not to have that power that it should? I would get a few hesitations & sometimes it would cutoff while driving. I did the vac test with the oil cap off while the car was running & it was trying to cutoff, so I'm assuming I don't have any vac leaks. I can mash the gas & it seems to have probs picking up speed. It's not coughing but not picking up like it should. The car has new plugs etc. It's not back firing. Could the HB be the prob......

 

the harmonic balancer(on z's) is just the crank pulley, and no it really does not have a adverse affect on performance

 

Yes it can cause problems!
I have seen balancers cause driveability problems on a few Chevy Cameros but not on a Z yet. It is possible though...the rubber deteriorates and spins the pulley to balancer slightly. This makes it hell to get timing right and it also doesn't dampen the crank vibrations like it should and can even make it worse then running without one due to the balancer now being out of balance. If the rubber is wasted, don't hesitate to replace it. This may not be the cause of all the problems you are experiencing, but can be a major player in poor driveability.
Good luck!