The only weapons I've seen the terrorists armed with are AKs, RPGs, AKs....and more AKs.
This will be a Tom Clancy style war - most of the work will be through intelligence, satellite recon, spies, etc. The combat infantry part will be with small groups of SFs, not masses of standard infantry.
With all the SF people who will be doing night raids the new weapon will come in handy. Night vision is a must for night raids, and that 20mm will work well against all the trenches and caves found in that part of the world.
I don't think the weight of it will be that big a problem since they'll do most of their traveling aboard helicopters, not marching. Besides, these guys all probably lift weights in their spare time - so would an extra 15 pounds really make a difference?