Hi
Does any one know the max travel I can get away with on a whyte t129s

20mm more than it came with. A.general rule across most frames unless stated otherwise. Which should give you more than enough unless your thinking of downhill action.

Obviously your effecting the geometry that the makers of the bike spent a long time thinking about.

Depends on the fork, the Axle to Crown is more important. Forks of the same length can have 10-20mm different A2C lengths.

This is true

I would ask Whyte personally.

How well it works varies though, some bikes suit extra fork, some actually suit less.