If you do highway driving often, you may get tired of the 4.56 gears...at least I think I would. I now have 4.30 gears....BUT, I have a Viper T56 with a big .50 6th gear....I can cruise at 75mph at 2100 RPM's.
4.10's minimum though...I'd say 4.30's.
3 things I'd do right away:
1. replace the mid pipe...off road pipe preferably.
2. aftermarket shifter, such as a tri-ax or pro5.0. Prevent missed shifts and safer on the trans too.
3. a set of drag radial Nitto 275/40/17 tires.
First time to the track with the IRS...I bet your first pass is in the low 14's....then down to a 13.7 or so. It takes a good driver to be able to ride out the clutch and not wheel hop (which will/can destroy your 1/2 shafts) and get low 13's or high 12's with a bolt on full weight Cobra.
To get 12's n/a you will need full exhaust, CAI, pulleys, MAF, tune, sticky tires, and the balls to rev it up (pray the clutch and 1/2 shafts hold) and go for it.