About a month ago I tested all 3 of the DA Polishers we currently carry using 6.5" pads on a flat hood and as long as you use good technique, that is hold the pad flat to the surface, they will all rotate a 6.5" pad.
Heck Meguiar's did their testing with 7" pads and launched a new pad line for use with their DA Polisher and my experience with the company is that Meguiar's doesn't launch a product till they have all their ducks in a row. Sometime mistakes are made but they are very good at doing their field testing before launching any product. The 7" Softbuff pad is a dual purpose pad as it was also designed to be used with rotary buffers, so a compromise was made on size and thickness to try to provide a pad that caters to different markets.
Pad Saturation
That said, as a foam pad becomes wet or saturated with product, the combination of foam and liquid will act to absorb and dissipate the power coming out of any of the 3 DA Polishers but the Griot's will do better at maintaining pad rotation if all factors are kept the same.
Thin is in and smaller diameter pads
5.5" pads, and specifically thin 5.5" pads work better to take advantage of the tools unique drive mechanism but thousands if not hundreds of thousands of cars have been buffed since the PC was introduced from the wood sanding world to the paint polishing world since sometime in the early 1990's.
About a year and a half ago I posted that the Griot's DA Polisher kind of works like a
safe version of a rotary buffer in that if it's able to maintain the rotating action to a pad under pressure... well that's basically what a rotary buffer is doing. To a lesser degree that could be said of any DA Polisher but there is a power difference between the 3 popular DA Polishers.