Wow!
That is a lot of AP's! You are not running them as repeaters and the antennas do act as amplifiers, so you have medium power.
Your distance is about 1.8Km total?
Each router/ap is about 20Mw output.
Your antennas are doing all the work, I calculate 640Mw ERP after the 24Dbi antenna.
The snow will cut down on the signal and cause you to loose packets, as well as trees and even humidity at that distance.
You say you had 10Mbps before you put the dap-1360's in? What mode where you running the old equipment in?(B,G or N?) Also what wifi channel.
Do you have line of sight? (can you see where you need to go CLEARLY from where the internet is?)
If it worked well before(the high gain antennas will easily make up for the low power AP), I am leaning towards the DAP-1360's being configured wrong.
B mode is much more tolerant to range(in most cases) then G or N. Also with their being snow, this COULD cause packets to be dropped, B would also handle this much better.
Is the signal strength solid or does it wobble and change? At 60-70% you are right on the edge of rolling back to a slower mode.
Lastly are all the AP's on the same channel and are they interfering with each other, newer AP's are programmed to NOT interfere. With this setup you need to use 2 NON overlapping channels, 1 for each wifi point. Please fill in the channels where the question marks are below.
inet<--->Clavister SG50<-cable->dwl-2100ap<-CHANNEL??

->dwl-2100ap<-cable->dap-1360<-CHANNEL??

->dap-1360<-cable->dir-300
Thanks!
Patrick