Car is booked in with BMW to take a look at the surging above 3000rpm (research online suggests a reflash may sort the issue, fingers crossed it's that easy).
In the meantime my DICE Mediabridge arrived!!
Honestly every E85/E86 Z4 owner should buy one of these, it works so well, and adds so much functionality for such a small amount of money!! Simply disconnect the CD changer (just peeled back the boot carpet to access the two cables), connected them to the Mediabridge harness, and it was ready to go. My only complaint would be the bluetooth audio streaming is a little on the flat side, it doesn't pump the bass nearly as good as connecting via USB, and it seems to clip the highs as well.
As it stands, I connect the iPhone with the USB cable (which also charges it) and listen to music that way, while calls come through over Bluetooth.
My iPhone connecting:
USB/iPod mode. Works great with music and TuneIn on the iPhone. Shows track names on the screen, and next/previous track from the steering wheel.
I'm told phone call quality is excellent, even with the roof down.
All in all for AU$300 I'm stoked!