Hey there, Foga! Welcome to the TS:R Forums!
Like you, I have been long fascinated with cut content and other features that may have been present during the early stages of development for a game but were not included in the final product. You're right, TimeSplitters does indeed have content that can be found in the game files on the disc but cannot be accessed directly through the game. Here is a video that features textureless weapons that can be used but only through an Action Replay device on the Gamecube version of TS2:
TS2 Action Replay
This website will show you images of weapons from TS2 that looked different during the Alpha/Beta stages of the game's development, as well as weapons that aren't in the game at all!
TS2 Beta
Also, I bought a PS2 Demo disc last summer that features an almost fully playable demo of TS2 Siberia! The demo ends right when you get to the top of the damn to confront the gunship, though you can still run to the first gun turret, which looks quite different from the final version. Here is a video of the demo:
TS2 Demo