RF LOAD above the treshold will lead to bad drops. TCH is more resistant than SDCCH (thanks to AMR), so that’s perhaps why you’re seeing more SD Drops.
If AMR is not activated, then you can assume that people in the most interfered areas are always getting a SD Drop (or almost always). Statistics will show high SD Drop Rate. But TCH which are already allocated might be kept alive more succesfully, because people who managed to get a TCH are located in less interfered areas.
GPRS is of course impacted by radio frequency planning ! It’s even more sensitive than GSM !
If some cells don’t show bad QoS, it is because :
1/ they are not fully loaded (not all TS are busy at BH)
2/ they receive less overshoot from other neighbor sites.