Originally Posted by mults

The only thing that I can think of is that the new exhaust is closer to the clutch hydraulic line causing the fluid to boil. If the fluid is old, and the new exhaust is closer or puts out more heat, that heat is making its way to the hydraulic line. Try changing the fluid to a higher temp one (Motul 600) or place a shield between the exhaust and the hydraulic line.

These car have a notorious habit of boiling the OEM clutch fluid. Mine did it once at the drag strip and the pedal went to the floor for a few seconds and once the heat dissipated, the pedal went back to normal. Keep us post as to what you uncover.