Going to freshen up my 500 s and I have been looking at there heads and cam upgrade kit with ecu reflash. They claim 600 hp.

There are a few threads on here regarding this kit. I was looking at it on a previous cat I had with 500 efi's.

What I found out was that 600 hp is tough to acquire from a 500efi without changing the intake plenum & TB. The small throttle body located at one end makes it challenging for the larger heads to perform with restricted air flow.

Do a search on here, you see some results from people that actually tried it. You can also contact Eddie Young, he is very knowledgeable and great to answer any questions.