Hi Stuart,
Are you using 944 torsion bars? Which ones?
A rear stabiliser is used to restore the 'balance' after fitting an uprated bar to the front.
The Karmann Bugs are much heavier than their tin-top counterparts, and despite all that extra steel at the bottom, the hood, frame, padding and any moisture content possibly lead to a centroid slightly higher up in the car. This makes a rear stabiliser a must.
However, fitting a rear stabiliser to a car that retains the stock bar at the front I fear may induce poor handling on a wet road.
As you know, I have a similar set-up to yours, and fitted a 23mm bar to the front and used the car for a couple of weeks before fitting a 16mm bar to the rear. The handling was 'unpredictable' to say the least.
There is a chart in one of the tech articles that tells us where to find the different diameter 944 rear bars, I believe the Kerscher rear stabilser is an oe Porsche unit
I'm HAPPY with my 23/16mm Kerscher combo
Matt