You said((the bandwidth of the GSMK filter is 67.5 and the output of the mobile is at 270.333 .Divide 270 by 67.5 you get 4 i,e you would need 4 channels in UL and 4 channels in DL thus giving u 8 timeslot per TRX)).
So there is inconsistency, Cuz 67.5 is Hz , & 270 is kb/s.
Thus 270 is just compatible with 67.5 to produce four status on I/Q modulating diagram (00, 10, 11, 01).
Regards