I played a PF800 a couple days ago, and it sounds pretty much identical to the PF500 to me except it has more headroom, which is always welcome on a micro for those who play loud. Haven't directly AB'd it with the 7 Pro but I have directly AB'd the PF500 and 7 Pro, and your guess is spot on as far as I'm concerned.
Is it worth the extra scratch for the 7 Pro? It would be to me if I were in the market for a high power micro, but I've heard of plenty of folks who prefer the PF500's sound over the 7 Pro, so that's a judgment call.
Ampeg Portaflex Club #1