Can you feel the pedal pumping at all under your foot even at relatively normal braking? If so (and assuming you dont have ABS) then it probably is warped discs. Get the run out checked first though before getting a new set. If you dont feel any judder/pumping then it is more likely brake fade.
My 214Si and my 105 both suffered terribly with it even from some quite modest speeds. The EBC turbogroove and greenstuff pads have gone a HUGE way to eliminbating that problem.
£130 for a set of front discs and pads.
under normal breaking it seems fine (never done it).......just coming down from 70mph or more........the pedal almost feels like abs is pumping away (but not got that!) but the shaking goes thru stearing wheel too!