Okay, there are a few things that can be done to improve for cqb.
The hop up nub on every CM 16 (including the 2 I own) is to soft and doesn't push the bucking into the barrel enough. You can place a small piece of wire into the nub. This will increase your effective range and improve accuracy some, or you can replace the nub with a flat or bridge hop nub. This will also require a flat hop bucking. The flat/bridge hop should gain more accuracy and effective range than just stiffening the stock nub.
The best inner barrel is the straightest, smoothest one you can get. Barrel diameter and size isn't as important, even though you'll see many debates on the issue.
Personally I wouldn't mess to much with the rate of fire on a combat machine. It may handle it okay but I'd be inclined to believe that a catastrophic failure is more than likely going to be the outcome. But if you must, there are a couple of ways to do it. I won't go into detail since I don't do high ROF guns (maybe someone with more knowledge will step in).
More power i.e. Lipo.
Now having said all of this. The hop up / barrel changes are easy to do and could see the most difference without causing more stress on the gun so I would recommend starting there, gaming it and seeing if that's a good 90% solution or if you wish to continue into the mechbox for more upgrades.
Hope this helps and if anyone else see's anything I'm way off on please correct it.