So, what's a tuner and what's it do? It reprograms the engine's computer, mainly to advance the timing (and thus require premium gas), moves some of the fuel/air/spark settings from the 'good mileage' end of things into the 'good performance' corner, and (if you have an automatic, which I don't) changes the shift points to higher RPM's. If you want more detail, there's a brief article over at Wikipedia. Here's a video of the net result, showing the same car before and after. The manufacturer is claiming an 18 HP increase, but if I'm doing the kinematics right it looks closer to 24.5 HP for this case. Maybe the guy just had a suboptimal baseline run, making the tune look better than it really is. And while I have not dyno'd my car since the upgrade, I assure you that the difference can be felt.
Now for the burning question: if the 2011-onward V6 puts out 305 HP, and the 2010 GT V8 puts out 315 HP, who's going to win a quarter-mile drag race? Answer here. Nice test procedure and a nice video, except for the music overlay. What about if we repeat this after a few common mods to the two models? Two videos of these results, here and here.
OK, that's all the hotrod action for now kids. Eagerly awaiting the strut tower brace (backordered), but that's more of a handling thing. After that, maybe tires next year and that's about it. But remember: leave this stuff to the