I did the m50 and I highly recommend doing it! I lost a bit of torque, but it was barely noticeable, but then I installed underdrive pulleys which made up for it. The gains up high are amazing! From about 3.5k to 6k there's a HUGE pull that wasn't there before. It gives your car more power at higher speeds too, like cruising along the freeway at 80, if you punch it you get a nice little kick to move forward, after you're already moving that fast. Sure you loose a little off the line (no joke, it's to the point where you barely notice it, if at all), but the gains up high are very worth it imo.