Dear Sam,
If u use only CS1~CS4(9.05~21.4Kbit/s), for better data rate u have to ensure better radio envoroment.U can also modify the parameter setting GMANMSAL to control the no of MS per PDCH in the cell.
If u have EDGE license u can enable EEDGE parameter & improve throughput(data rate) by assinging coding scheme MCS1~MCS9(Upto 59.2Kbit/s). By using parameter GPDPDTCHA u can dynamically use cs ts as ps ts. u can assign value 100/50 depends using 100% or 50% of free TCH.