Well, I'd highly suggest getting a GSR. People might say the LS is better for boost or what have you, but it's not really true. It's just easier to run MORE boost, because of lower compression ratio. With the GSR, you'll get more power per PSI, and you're starting with more power already.

The tuning should be done on a dyno by someone who knows what they're doing. I'd just rent one out for an hour and have the place tune it, be sure to find a reputable shop though.