Fahim,
Sub-optimal because the BCCH carrier will send some timeslots in GMSK (max power) and other timeslots in 8PSK (with a power which is a bit less than GMSK).
So neighbours MS will measure this particular with a level which is somewhat lower than reality. (measurements are performed on any timeslots of the BCCH carrier, not the TS0 only)
Usually, the delta between GMSK and 8PSK is less than 2dB… it’s somewhat acceptable 🙂