I am also in doubt, but in the largest accounts it is not so important because AGCH and PCH are shared a block by block basis.
I think that you may also check out the following:
1)The DRX mode of the MS (DRX-mode or non-DRX mode after packet transfer->packet idle transition and non-DRX timer), if DRX mode is supported in your network. Really Immediate assignment Command on A-bis has no Paging Group info as in Paging command. and in DRX mode an MS is looking for only it’s own paging blocks (and not for AGCH blocks, defined by BS_AG_BLKS_RES). In non-DRX period after packet transfer->packet idle transition it observes all CCCH blocks in it’s CCCH group.
2) Initial timing advance estimation for DL Ack from MS. Are you send TA in Immediate Ass? if no, then you must indicate to MS to send PACKET CONTROL ACKNOWLEDGEMENT in 4 random access burst form.