the ODPDCH won’t have much effect in E/// network, because it is set to a very high value by default (100%).
try modifying the parameter that allows to pack more TBF onto the same PDCH (one parameter for UL, another one for DL). By default it is 2 ! Try 4 or 5. That will strongly reduce your GPRS Load !! It will also reduce the GPRS throughput, but – due to the bursty behaviour of data traffic – I think it is acceptable.