Thanks Goatwuss - unusual name by the way must find out how you got it. My research so far has reinforced your advice.
The custom installers are looking an average of 4K for a system using their experience. I cannot warrant spending 4K on a ragtop due to its limitations.
This lead me to visit all the BMW blogs to see what other M roadsters / Z3 owners had done and most say the compromise for the money is to keep the locations for the main speakers since this was engineered for the best audio in a ragtop, but upgrade them. The biggest upgrade for the money being the Amps - One for the main speakers and one for the sub. This is then followed by changing the 5.25 inch kick panel speaker and finally the door mounted treble/mid speaker.
So the amps are easiest for me to select (power ratings, matching) then the speakers,
So I am still after recommendations for Speakers.
I order of preference of other peoples systems I am leaning towards AVI MXD 130, MB Quart Premium line, Polk SR250 series.
Any other recommendations for speakers ?