Just to give another solution:
Retransmissions in case of high MCS is because the C/I is just not good enough. So in those cells, you could simply limit the max MCS to MCS8 or MCS7 instead of MCS9.
The problem in EDGE is that the link adaptation is not configurable : the thresholds to increase or decrease the MCS are static.
Sometimes, I guess those tables are a little aggressive, and the MCS chosen is too high compared to the actual C/I.