The engine has been extensively modified. Don't change what's not broken, but modify what you want changed.
The point is: An old engine is an old engine.
It can display graphics to a certain amount of quality with an acceptable amount of hardware consumption. If you try to exceed the limit, however, it becomes a hardware muncher.
See the Source Engine, for example. It really can't do any better than Team Fortress 2 without going rampage on your hardware.
The idTech 4 Engine is about 7 years old now. Call of Duty is a very good example for the limits of that engine, wether it be heavily modified or not. Go any further and it won't work properly anymore.
If you use a recent engine on the other hand - the Cry Engine 3 for example - you can create equal graphics with a lot less hardware strain.