well UT2k3 uses a different format entirely to UT as its a totally different engine.
all of the TR upto 5 although they've used different graphical engines they've all used the exact same game engine. The reason is so they could release sequals quickly (1year dev per game), each time they decided to add something to make it slightly different to the previous version.
the difference between TR and TR2 worlds is just the header, there are no other differences... between TR2 and TR3 are a bit more apparent as they changed the engire backbone of the relationships of the portals - which means although you can load a TR3 in TR2 it'll look like someone has taken the world and shaken it up
really if you think about its alot like Quake BSP, the format itself didn't really change dramatically until 3 and even then the core geometry stayed the same to the point where you can actually load Quake1 & 2 levels in 3 - you have to bypass the wrong version checker error & add support for the PCX/WAD/WAL/PAK formats but the BSP itself will load perfectly fine
i have a copy of the TR2-4 formats if you wanna have a look over them, i did't backengineer i just nicked it from Core before i left (hehee) it'd be pretty simple to make a loader to use the format in DBP