I bought a powertap for my road bike (dura ace - 10 speed). I would like to use it as well on my TT bike (Ultegra 9 speed).
I have two options:
1) Swap cassette/shim each time I ride the TT bike. Cheap but time consuming.
2) Upgrade TT bike to 10 speed. I have a spare 10 speed cassette so I would need 10sp bar end shifters and derailleur (and chain). More expensive but easier, just remounting the PT and running the cable.
I ride the TT bike a 1-2 times per week (not including races) and race it about a dozen times per year.
Third option of buying another PT is out of the question and 4th option of STFU is not of any help.