This is my 996 powered boxster S...parts courtesy of Techart, Gemballa and the engine swap I purchased the engine and kit from "fast" willy over at FVD....Mr. oh ja it plugs right in...RIIIght ....anyway your thinking is right on looking for GT3 convertible .... I am thinking though of a 991 C4S with powerkit or if I really cant control myself....give in (which from an image it would be like  Darth Vader telling Luke that he is in fact his son...and luke screaming NOOOOO) ....anyhow I tried one of Rufs supercharged 996s C4s cars that my friend modified the suspension on and man that thing was smooth and smoked a turbo of the same year.....As a value proposition ...its not there --but as something that I am hoping would really be a hoot...that is a convertible with two trunks...cost of the car imported and taxes paid in the US is $162,000 which what I want on it...turbo S here is $188,000 and C4S cabriolet will be $150,000....