Hi, just thought I'd post a quick update... I took the plunge and installed the CM12.1 nightly (for the 28th) and installed rSAP for Android. It took a couple of attempts to get it setup as I was using the built in SU capability for CM12.1 (need to enable developer options) but, everything seems to be working as before.
I haven't done a journey for longer than a few miles but the car connects via rSAP and all the data features are available and there was no drop out at all. Phone book was also transferred.
Hopefully anyone else that wants to try CM12.1 on the G3 has the same success.