My experience with Potomac Pedalers is that they absolutely will drop you, but they give you a cue sheet so you can find your way back.

I did a "C" ride, 12-14 mph, and in the end my average was 13.7 mph, but I was way at the back of the pack with a few other stragglers. The average of the folks leading the ride and most of the participants had to have been considerably over 14 mph.

It was a nice ride, but I never did another one with PP.