Retransmission and set MCS on SM's

What is the acceptable retransmission percentage of an ePMP SM?  We try to keep everything below 10% on the uplink and below 5% on the downlink.  

Second question, if we set the MCS on the uplink modulation of an SM, does doing that affect the performance or capacity of the ePMP AP?  Obviously this affects the uplink capacity of the SM.  We have a 2.4Ghz ePMP site that has terrible uplink retransmit rates on the uplink and I was going to manually set all the MCS rates on the SM to bring down the retransmissions. 

in a way, it does affect the total uplink achievable. 

the EPMP works differently from other wifi driven gear.

in the event that all of your airtime is being used, each CPE gets an even share of the airtime (assuming they are all the same priority)

if you have all of your cpes stuck to MCS12 for uplink, and you hit 100% airtime then your AP will only uplink of 6mpbs in a 10mhz channel, 75/25 fixed frame.   if you have half of your cpes in 12, and half in 14, and you're moving the same amount of information as just mentioned, you'll have a little left over airtime for additional bandwidth. (an average of 7.5mbps in the same frame channel width and ratio) 

retransmissions being high are sometimes fixed by downgrading the mcs rate, BUT only when the burst noise that is causing the problem isn't loud enough to interrupt then required SNR of the incoming defined MCS.  

for example, if your floor is -90, and RSSI is -60 and you have burst noise of 78db.   your CPE may try and use MCS15 with a 20 percent retransmit rate because the noise occupies 10% of the airtime.   if you force the CPEs to run at MCS11, then you only need a SNR of 16 (drawing from memory so that number may be off some, but it works for an example)       the catch 22 is MCS 11 is less than half the data rate of MCS15 so you'll end up losing total throughput doing that.    

are you running into problems you believe to be caused by a high retransmit rate?  aside from the obviously lost potential max speed.