If you are mainly looking to improve acceleration performance, and/or shifting behavior, the SuperChip will be a big help for you. Firms up the shifts and greatly reduces the difference in 0-60 over the manual gearbox. You'll have to run pricier gas, but should get some of that back in improved mileage.
If you want to stick with running cheap gas, I know there's a shift kit for the atx, but I don't know of any here who've installed it.
Hth,
dan
