First of all Ted, welcome over from the FalFiles. I like the M4, but I also like the dissipator. The dissipator has the advantage of being a "true" carbine. As you metioned, the full length sight radius gives the shooter with iron sights an advantage over an M-4 or Car15 shooter. I like the M4's ability to be shortned to accomidate being carried in a vehicle. If you have the 4 position tele stock, there is a hidden advantage over the A2 stock. You can taylor the pull or length of the stock to you. It also accomidates body armor, LBE/V better because you can again adjust the length.
Another problem, all of the dissipator barrels are currently HBAR's. This helps dissipate heat, but for CQB it will be a bit slower than an A2 configured M4 barrel.
I think the perfect CQB/Carbine would be a dissipator model, with a light weight barrel and good FS/muzzle brake. A1 upper with AO A2 sights, and an A1 length buttstock made of A2 material so that it presents faster, and accomidates body armor better. The A1 stocks are out there in small numbers. Colt produced some for the LW Sporter, and Canada has them available for the C7. BTW the Canadian stock is made on the Colt mold-the letters CS are on the right side near the junction of where it joins the receiver.