When I got my Z4 last year it spent ages in the dealer and although they gave me loan cars, none were convertibles, meanwhile the sun was shining...I was paying for a Z4 but driving a 1 series with dealer advertising all over it! They dont seem to realise we buy these cars for the weather!
Unless you are prepared to walk, then I wouldn't bother calling....you need a decent "threat" otherwise they wont care, but obviously, if you threaten to walk away, you need to be prepared to do it.
One thing that springs to my mind is to ask to have a loan Z4 for the interim period. Perhaps not of same spec, but a Z4 all the same. BMW allegedly have such "loan" cars available. They offered Carol one I think when her car was being re-ordered. I dont think it materialised, but think its a possibility.
So perhaps put it to them that you want such a loan car until yours arrives, or you want a refund and then go elsewhere. Give them a few days to sort it out.
Just a thought...