Interesting thread, thanks chazstace.
I'm in the UK with an '04 Coupe and also have poor FM reception since the Dynavin install, and also non-existent TMC with the supplied aerial. At least I know it's not just me now!
I've actually been considering a roof antenna. I have a standalone DAB tuner feeding the Dynavin's aux-in, and already have a dual-band DAB/FM roof antenna on my other car, an E46 Touring, which works very well. the Coupe's lines would be spoilt more I think by a rod on the roof, which puts me off. If I did though, maybe I could use the rear window antenna for the TMC.
Or maybe I need to practice my soldering skllis and build my own antenna amp...