I have the OEM bluetooth factory fitted to my GTD and I did have the RCD 310. I have since retro-fitted an RNS 510.
The BT works fine with the RCD 310 in a more limited capacity than the RNS. When you receive/make a call the radio will mute and "PHONE" will appear in the radio display. The call audio will come through the speakers. The MFD though will display everything you need - phone book, call register, etc, so to answer your question, the highline screen in conjunction with the steering wheel buttons is enough.
Also, BT audio will work with the RCD (e.g. you can play your iPhone music via bluetooth).
The RNS does give much better phone integration though!