There was a crossover point at which Porsche changed the mounting. On all "S" cars and 2000 and later non-S cars, it's easiest with the bumper on. For 1997-1999 cars, it's basically impossible to do with the bumper on.
I drop the exhaust fairly frequently for competition, and I have never once removed the bumper on my 2000 S. Piece of cake. Do not attempt on the earlier cars!
