it cant be a full "custom" tune unless they strap your car to a dyno and tune for your car.

You can put 2 srt6's side by side with the same mods and same tune, both will run differently and make different numbers.
Also, having a shop that has done a tune at 5000 above sea level, then flashing your ecu and sending it back, where you live at 1000ft above sea level is going to make things way off.

See where im headed?