I have the SCT Xcal2 on my 2004 S281-SC and it is awesome...really woke it up from the PowerFlash. As far as I know, it is the only handheld tuner that works with the Saleen PowerFlash. The original Xcal, Predator, and others won't work.
You only need to provide the guy the numbers on your computer if you want a custom tune from him...which I really wouldn't recommend. Same place I purchased mine (Blow-By Racing via eBay) asked for the number and offered me two free tunes, but I declined.
I would recommend just buying the tuner and taking it to your local dyno shop that deals with (or knows how to tune with) the SCT Xcal2. It took my shop 1.5 hours to get my car tuned to where I wanted it (11.5 to 11.8:1 A/F at WOT). The mail order tunes are cheap and close...but I wouldn't want to risk it with my car and the amount of money I already have invested.