Uh, all the textures, static meshes, actors, blah blah, that are used in UT3 can be opened and used in UnrealEd for map creation. A lot of great mods (Jailbreak, TAM, etc.) are only new gameplay sets. As far as TCs (total conversions) go, the mod groups getting picked up devs put in some pretty damned hard work to get their game anywhere near that professional look. Which is why they get hired. And if you think mod groups these days can't put out fantastic visuals, look at Dystopia for Half-life 2.
If/When Epic starts their Make Something Unreal contest, I think you'll be eating some crow.