Could I get away with 100mm travel on a Tufftrax? The last forks had 80mm but there is 15mm worth of spacers under the stem - if I removed the spacers when fitting the new forks the overall height would be about the same?
Spacers under the handlebar stem don't alter the geometry, but longer fork blades/legs do. It might be OK, but a fork with a longer axle to crown length will reduce the head angle, and increase trail, albeit very slightly with just 20mm increase, which will generally make for slower steering.
There's also another issues, which is wheel flop, which is complicated to explain: http://en.wikipedia.org/wiki/Bicycle_an ... e_geometry
, but definitely something you don't want as it makes for very odd feeling steering.
You might be fine, especially as with suspension forks, once you take sag into account the difference between 80mm and 100mm of travel might only be 5 or so mm with the bike in a neutral position.
How sensitive you are to the changes might also be a very personal thing, I've seen a few bikes this forum with the "wrong" forks and people seem perfectly happy, whereas I fitted a set of rigid forks to a frame that were 15mm longer than the originals and I thought they felt awful, a 200 meter ride up the road was enough to know that there was no way I could put up with them; I think wheel flop was the main problem as the bike "fell into corners", especially at low speeds.