The quake 3 engine pushes a lot of polygons considering it doesn't off load the lighting to the GPU. It's probably about as good as it gets for the non-DX7 T&L generation -- well more like L.
The thing with the Unreal engine is as follows, the engine used in Unreal Tournament was basically a hacked up software rasterizer. It spent a lot of time culling. This meant it was very CPU bound. I believe this is the Unreal engine v 2.x
Unreal Tournament 2003 is version 4.x. This engine doesn't spend so much time culling, so it's faster in that regard. However, I believe Tim Sweeny's goes by the belief that the compiler will handle the implementation optimizations and concentrates on algoritimic (more design) optimzations. The thing is that algorithmic optimizations earn you a lot of performance, but there is a lot left behind. When you have lots of AI, Physics and all that going on you bog the CPU down.
The other thing to note is the butt load of geometry that is being pushed, this is probably causing the XBox to choke, since it doesn't have enough RAM. I believe, in outdoor areas the amount of geometry just makes the Xbox start calling on the information stored on disk since there isn't enough space in RAM. This sucks really bad thanks to the fact that IDE sucks up lots of CPU power. This is probably just another big reason for geometry compression through HoS and what not.
I think it comes down to the fact that the Unreal 4.x engine is a little bloated for the Xbox. But that's okay so long as you don't release a game with the Unreal 4.x engine for the Xbox.
