This has been covered a lot, but here's my .02:
you might find the 20" A2 style the worst of both worlds when it comes to scoping. Obviously, you're limited to stuff that will be functional on the carry handle due to eye relief, etc; if you're looking for more than 4x or so, it might be difficult. (I don't have this setup, I'm just going by memory from some pics I've seen here that folks have posted). An ACOG might be your best bet here.
You might be happier just getting another flattop upper, 16", then going to town on that one in terms of optics and mounts. You won't lose any accuracy in the 16" gun at those distances (or any practical distance, really), and with a few $$ invested in some ARMS mounts, you can scope/red dot the new upper for distance AND CQB, and change the config with some quick throw levers.
I have a 16" I call my Swiss Army Rifle. It usually has an Aimpoint on it, but I can switch to a 3.8x12 scope in about 30 seconds...and then slip the bipod on there if I need it as well. I've gotten 1/2 moa out of that rifle at 100 yards with handloads and the scope, so it does everything I'll need an AR type rifle to do.