Hello fine folks!
We have a customer with a great LOS at ~10mi from an ePMP 2000 AP utilizing an omni antenna (KP Perf.). The SNR seen at their Force 200 CPE is around 20, sometimes inexplicably dipping to 17. The Force 200 is, as far as I’m aware, is the highest gain SM available. Is there a better solution for this long distance scenario, apart from migrating to a 90/120 AP cluster?
A 2’ (or larger) dish with a connectorized ePMP 1000 radio would be your best bet. The UBNT rocket dish works well. RF Elements makes an ePMP → UBNT bracket adapter as well.
Force300csm (no point in spending $$$ on a 1000csm when the force 300csm is only a few $ more) and a 2 or 3ft dish. Kp make a proline thats both reasonably priced and easy to use. Make sure you order a set of long cables as it doesnt come with any.
I have a 9mi shot from a kp omni (e3k-L) to a force300. Its tower top (145ft) to barn top (60ft) that is seeing -74 with 28 snr.
Snr drops are common, this changes with moisture levels and density. Your best snr will be just before sunrise and your worst will be at sunset. This is because of the amount of difraction that is observed throughout the day. During the day it can move at random for the same reasons as wind currents move the moisture around.
I very much recommend against using AC client (Force 300csm) unless you are planning to start moving to AC in the very near future (if I had it to over I think I would have stayed with 2k APs and waited for AX). In our experience the AC radios do not do as well as the N radios on links that are already stretching the limits.
Also if you are not running v4 firmware on your 2000 AP you would have to upgrade to v4 in order for it to work with an AC radio.
Yes, you’ll have to have them upgraded to V4 for them to cross-communicate. However, if you are not upgraded to version V4 already, the question would be why not? 
I recently upgrade a ePMP2000 to Force200 link from 3.5.6 to 4.6.1-RC14 and the throughput went from 115 Mbit to 195 Mbit. That’s more improvement than expected, so probably a couple factors at play - but it’s not uncommon to see 10%-25% increases.
We still have some of our ePMP2.4Ghz gear on 3.5.6, but other than that - all 5Ghz gear is all running 4.6.1-RC14 and it’s all running better and faster than before.
I haven’t experienced the issues you’re having (DFS was a different matter but we got that solved!)… That said, we’re ensuring very strong SNR with Force300csm and Rocketdish LWs or Mikrotik mANT 30s (the latter are REALLY good). On the “stretched” ones you could force lower maximum modulation rates so in essence they’ll emulate N radios.