If you are unsure or find a flat spot somewhere then a Power Commander will make things perfect and maybe even better than standard. Stock maps are often made to run lean around the point the emissions samples are taken. A P/Commander will fix this.
My advice would be to take your slip on with you to the rolling road and do a test run with stock pipe, fit the slip on on and compare maps/fuelling, BHP and torque figures, this will tell if it's necessary to buy a Power Commander and see what your true gains are. Plus you'll have a print out for your cave wall.
