I have just dug up some US sales figures (October 09)

Whilst convertibles make up about 40% of the sales in the 911 range (and only 38% in Boxster/Cayman range) they represent less than 20% of all Porsche's sold in the US.

I suspect a large portion of the convertibles are California sales.

They do not sell well in Africa either (not sure about ME) for obvious reasons.

Anyway, the point is if you want to gain unit sales quickly, convertible models (especially 4 door versions) don't seem like a very logical way to do this.