This is a common and expected thing that happens with any replacement PCM. Will even writes about it in the packet he sends with his tunes, if he still sends them.
Even if you get one from another company, this is still something that happens regularly, the PCM has to learn the "variation" in the crank position sensor on the motor so it can get the timing right.
Look into an Aeroforce scan gauge under Gauges on zzperformance.com.
It's about 250 dollars after shipping, but if you have a tuned GXP, that'll be really fun to have anyway. It's a scan gauge that gives you a ton of operating parameters including Knock, Intake Temps, Coolant temps, Ignition Advance, etc... full list is on the site. More to the point it can do a case learn, so it's a good investment in having a tuned vehicle and will help you do what you need to do on top of just being a cool gadget.
Oh, it also reads and clears OBD II codes so you'll always have a code scanner with you too.
As a 2008 you'll want to get the "CN" model gauge.
