I our very popular friend "Twister" had a cam-only thread. He had some details.
http://forums.viperclub.org/threads/649972-Final-testing-of-roeracing-710-cam-expectations-exceeded
He stands out because he is the only one who chose to ignore all vendor suggestions about pairing better flowing heads with an aftermarket cam. He still made some decent gains, but is probably missing about 75whp from a set of better heads.
If you're going to go with upgraded heads, then a before/after on just the cam (with the stock heads) won't necessarily tell you what you want to know though. The cam that works best with OEM heads may not be the best for the flow of the aftermarket/ported heads.....
Good luck. but as others mentioned...just do heads and cam the first time. put down 570-600 rwhp and call it a day.