Plat does not conduct as well as copper. It is harder though (it lasts longer - nice for 100K mile tune-ups on cars that specify plat plugs).
For performance, would you rather have soft compound tires that grip well but don't last super long (copper plugs) or cheapo hard tires that don't grip but last forever (plats)?
Enjoy the coppers (Autolite or Motorcraft are my choices, though I've read good things about the NGK's in 5.0's. I always used them in the sports bikes and they worked well).
Good luck.