good choice on the base ... since its all black you won't have to paint the trim like on the limited (so out of place) ... and if you want leather you can buy SRT-6 seats for $400 dollars and have better ... the TPMS system is a big limiting factor on changing the wheels so that's one less piece of trouble for you.

the speedinnovations chip adds a real world 19 hp / 19 tq so that will be your first bet ... magnaflow just came out with a exhaust system and the intake system can benefit from drop in K&Ns or larger air boxes altogether such as the ones found on the SRT-6 (they drop in nicely). Evo Sport makes a set of under drive pulleys.

that will probably land you at a +25 - 30 hp conservatively speaking ... can't guarantee even that as few have gone that far with the car.