it would take some serious braking to warp the discs.

i'm having the same problem with my turbo. it shakes to fuck under braking. the discs are new, my tyres are fine and all of the bushes around the front end are fine so i'm now thinking it must be something to do with the steering rack.

is yours only under braking aswell? you'd know if it was your tyre balancing cos it would do it constantly.