My experience with Profoto has been that the specs for color, consistency and recycle times are accurate. At least they have been with my Pro 6, Pro 7, Acute 2s, compact 300s/600s and D1 Air 500s.
You were looking for 'low' power settings and 3 frames a second. Define low power and I'll test the D1 500s.
Since the rx600 has 6 stops I look at low as anything from lowest to 1.5 stops above lowest setting. Hence, using the 10 points scale, 1.5 is lowest on the RX600 and 3.0 is where I would expect the rx600 to start slowing down.
My experience shooting with the rx600 is that they do not keep up with the Canon's 5DII rapid fire set at 1/160 shutter. For every shot there is a complete miss fire, and that is at low settings and the same applies to other lights I use on the set like the rx300 and fx200ri.