I have just got mine and so have been doing research before I get it done.
Apparently a stage 1 map gets around 195bhp and 295lbs of torque, although a harder map can be done with higher torque but you will need an uprated clutch if going over 300lbs. As long as the engines run in properly then it's OK on low or high mileage as far as I'm aware although higher mileage will have a weaker clutch therefore liable to slip.
Hope this helps and feel free to correct me if I'm way off. P-Torque is very popular here if you are local or fancy a day trip!