I recommend the test pipes, underdrive pulley and filter as the best bang for the buck. 25 rwhp for less than $1000 and you might save a little if you find a used pulley.
Spend another $1000 on a tune and you should get smoother driveability and another 10 rwhp. Just updating the engine operating software to the latest version reportedly improves driveability and costs just a couple of hours of labor if you can find a shop that will do it. I would like to do this on my 08 but it is modded and under CPO so I do not want to take it to a dealer and have not found any independent shops in the NH/MA area that can do it. Apparently any shop with Autologic equipment can do it.
I spent about $2500 on an aftermarket x-pipe to which I added quad resonators, a used underdrive pulley, a K&N filter, and Vishnu Procede with water/meth injection and am content with the performance of the car now --power is probably up by around 15%.
If you think you might buy a supercharger, skip the tune and pulley since those two items would be replaced with the SC kit. The SC kit does cost significantly more, but you get so much more power that the cost per rwhp averages out well. It is also the only way to get big power in these cars. Spending that much simply is not an option for me at present, but maybe in a couple of years I will look for a used kit.