Firstly PC is indeed a cell level parameter. but PC algo stats thatBeacon frequency should always be transmitted with max power and no PC should be applied on it for reasons of selections and reselections for idle mode MS.
Secondly as Pix advised, Idle mode will be affected by the BCCH TRX and its level of interference, so even if all active TS are on hopping TRXs (not affected by BCCH TRX`s level of interfernece), all idle MSs under this cells’s coverage will be affected. loss of paging, RACH, AGCH is possible.
You are right when you get all active MSs on hoping TRXs you will avoid bad QoS, but generaly BCCH TRX interference degrades the cell’s performance significantly.