I've had a N/A 2007 Sly since Nov. 2006. When I ordered it in August the dealer couldn't tell me when I would ever see a GXP if I were to order one so I went with the base model.
After a year I got curious and took a GXP out for a spin and was stunned at the quickness.
At that point I asked the Sales Manager what kind of deal he could give me trading-in my pristine 4k, never been wet car. He said "the car plus $11,000. Dropped my fun-dial to the pin.
So, I had the Hahn Stage II turbocharger installed by a local performance shop. There were some problems with the ECM tune initially, but the local shop was able to iron them out. Now the car runs great. The only thing I had to do is replace some of the Hahn clamps with t-bolt clamps to keep them from popping-off. The dyno shows that my car has about 24 horses more at the rear wheel than the GXP.
I'm pleased with the set-up, and I saved thousands of dollars which hopefully I some day can put a piece of it in to some suspension upgrades.
If you are not a really good mechanic I recommend having a pro install the turbo-kit as this, to my line of reasoning, is the reason a few people have experienced problems with aftermarket systems.