Mark's exactly right. Sympathetic improvements from well-know sources will not affect a car's value downwards unless it's a pure, concourse, original.
There is a very small market for the "museum" cars, where collectors will pay almost anything. You can't ever replace or recreate a mint, original, low-mileage rare Porsche, and IMO these include cars such as the RHD turbo cab, the Silver Rose in it's full tartan splendour, or the few cup cars remaining.
We're going to see the emergence of a "restored" class of car now that they are all coming to the point where you have to be looking at both major new parts, and attention to the bodywork. Where these cars find a value, and where originality becomes less relevant and alternative "improvements" become more sought-after than original, I'm not sure I know yet?
I still have a sneaking suspicion that the truly original, unmolested cars will continue to rise out of proportion? The one thing you can never replace is the mileage, and that's an obsession with collectors of expensive older cars. A real shame, as the whole point of owning them is to use them!
So where does the lightly-modded turbo, with things like KW, fit in? Logic would say that you can buy a more modern car that will out-perform the 944 in every area for £15K, so why buy, or build, a £15K 944? To me, it seems that there is a type of buyer who fits a certain profile. The guy who had a Countach on his bedroom wall, next to Sam Fox? The alternatives are the other affordable classics from the same period, but in all honesty would you really rather run a 944 or, say, a Quattro or a Cossie? A GTV6? A Delorean? [&:]
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