Most radios can’t manage their top MCS rate when the TX is set very high. I have no idea of the max TX for ePMP3000L to get 256QAM@40MHz but if it’s the same as ePMP3000 its 28dBm at 5.7GHz which is quite impressive. The legality of doing this in your country is an important consideration.
For most WISPS, your subscriber units are usually receiving not transmitting so their TX and MCS doesn’t matter so much, just let ATPC do its job as you don’t want subscribers too loud.
If one of the chains is -80dBm that wouldn’t normally indicate either
- misalignment
- fresnel incursion (e.g. trees)
- client is outside sector azimuth or elevation.
- a faulty lead at the client end if it’s connectorised radio.
Capacity vs MCS? The answer is usually capacity but it really depends on your environment. Most of my towers are so busy or there’s so much interference from other operators that 40MHz channels are just something to dream about. I try and get as much syncronised as possible for channel reuse and make sure I use high gain subscriber units as the pick up less interference due to their tighter beamwidth