Which pipe for Kx500?

I had a FMF Fatty pipe and Powercore silencer on my 94 KX500, it definitely made it hit harder, can't say about the overrev though.. I preferred the stock pipe w/ Pro Circuit 304 silencer, I found the FMF pipe made the bike that much harder to hang onto for a 20 minute moto.

A Pro-Circuit pipe would be a downgrade, not an upgrade.
I bought my '89 KX500 used, and it had the PC pipe on it. It ran like it had a sock in the muffler. Slow and mello. I put the stock pipe on, and it scared the heck out of me. (The 89's did not have the heavier flywheel of the '93 and newer 500's.) I figured that I bought a 500 for power, so I was not going to neuter it with the PC pipe, but I had to do something. I added a flywheel weight, and was happy. Controllable but crazy power, like the newer KX500's.
I was at the USGP in 1990, and asked Mich Payton, owner of Pro Circuit, why he made a pipe that de-tuned the 500. He hesitated a little and then admitted that most people who buy 500's were scared of the power, and liked the power smoothed out. (Smooth meaning slow.)
The cheapest way to get different power out of the bike is with AVgas. For some weird reason, my KX500 would get really frisky throttle response with AVgas. With pump gas, it would be mellower, which was good for muddy days. With good traction, I would run the AVgas.
Also, I tried the optional thinner and thicker head gaskets. The thinner one gave much more low end, but overrev went away. The thick one gave more top end, with a softer low end.

My bike pinged like it was going to burn a hole through the piston, until I put the thicker head gasket on.. Smoothed out the bottom end and made it rev nicer, or so it seemed.

I'd get the Pro Circuit "Desert" Pipe (not regular), also with the 304-stainless silencer. It's a great setup.

More power out of a 500? Sheesh!

Check out the new KX500 pipe by Zip-Ty racing. It tucks in better and is ceramic coated. It's actually made by Flow, former pipe builder for Noleen. I just bought one for my YZ250.