We have a YZ155 ( overbore only, not stroked ). It is better than the stock YZ. It has excellent power off the bottom a revs cleanly through the entire rpm range. I don't think we lost any top end, though initially it seemed like it - I realy think it was the extra boost off the bottom that initially made it feel like it lost a little on top. We love this mod, I can actually carry the same lap times with it now as the 250F we have - it's a little more work than the 250F but at least it will run with it. The bike is way lighter than the thumper and has more grin factor. We had to buy the piston from RPM then send the stuff to Forward Motion - Eric did a great job with the boring and the porting - love his stuff.