I can't confirm this for the RX, but in general (for the Prius HSD) there is full assist in WOT. You get close to no assist below full throttle, and at very low throttle you get no ICE and all "assist". The reason this makes sense is that the hybrid's 0-60 time is slightly faster than the non-hybrid version (and 0-40 times would make this more clear) so that extra acceleration has to come from somewhere, and the 0-60 times are obviously done at WOT.

2001 Prius Photos       2005 Prius Photos
'97 Prius Simulation Prius inner workings