If you have an SECS/Motorsports Development coilpack board fitted, you'd do best either phoning Stu at MSD on Monday or dropping him a PM about the rev counter. I'm sure I remember reading that the 3dr rev counters are less likely to work correctly after being modified.

If the ECU doesn't have an MSD coilpack board fitted (it is possible on the P8 ECUs to run a coilpack without one of these boards) and if you're prepared to do some rewiring, then there is a modification to the rev counter feed that may work. Let me know if this is of interest, as I can't be bothered to type instructions now.

Alternatively, an 80mm Stack rev counter fits in the clocks fine with a bit of work and will definately work 100% - looks better too.