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A quick Google finds comparative figures for the 996 turbo, which is a different chassis but the delta is likely similar:
Coupe 27,000 Nm/degree
Cabriolet 11,600 Nm/degreeSo the lack of roof makes a difference, that's undeniable.
I believe the cab has softer springs, they're certainly cab specific (coloured rings identify them on the spring), I don't know what the respective spring rates are however, can vs coupe.
The Ohlins springs (for both) are 60 Nm front and 120 rear.
I wonder how much of the improvement was in the kit he chose, and how much was just putting new bushes, springs and dampers on. A V-reg car will have shagged-out suspension by now, as he found out.
I doubt he's going to buy another one the same and do the same job but one with brand-new Porsche stuff, but that would be an interesting comparison.
Yours being a convertible, and not so stiff, I expect you might need a more compliant setup?