With the lower pitch you've got better control of speed, as well as a better start.
I meant to add that after reread my post. The speed control is really excellent, at the the end of the season I lost my Perfect Pass yet again and had to rely on driving by RPM's even with well over 2K of ballast, lead, Battery and people it was no problem.
Art, how clean is your wave with the 911? I never noticed the difference until I damaged my 653 and had to drop the 515 back on for a few weeks but it was pretty big.