One thing I did find out while messing around with the android version is that if you create a whole new Aircraft without importing settings from one of the Aircrafts on their list it does seem to let you edit the performance data. But you start with nothing so you have to enter all the data for...
I know it's been quite some time but I was wondering if you figured out what was causing this? I am having the same issue with Garmin Pilot on android.
I contacted Garmin support and they told me they didn't know and that I would have to delete the aircraft and recreate it. Not really something...