I just learned that Porsche Classic does tire tests on older "classic" models (including 9x6) and compiles charts recommending tires (summer and winter) for them. These charts are only available in German and aren't even linked from the English downloads page, unfortunately.
But if you read German or can make educated guesses, they're still useful. Download them at
the Porsche Classic downloads page, under the labels Sommerreifen and Winterreifen (summer and winter tires, respectively). At the time of this post, the charges are dated March 2014.
Column headings are:
Recommended summer tire manufacturers and types
Radgrösse = wheel diameter
Profilbezeichnung = tire type designation
Rollwiderstand = rolling resistance
Nasshaftung = wet grip
Geräuschemission = external noise
Presumably rolling resistance is the inverse of grip, so the higher in the alphabet, the stickier (and less fuel efficient) the tire. For wet grip, the earlierer in the alphabet, the better. If you have, or wish you had, PSE or desnorkled intake or similar auditory enhancements, I suspect the noise rating will be of no interest to you.
According to
this site, the "specification number" is Porsche's version number of the tire. The scale starts at N0 (first version that meets Porsche requirements) and goes up each time the design is improved. It doesn't measure the performance in any way.
The note at the bottom says "Only tires of the same manufacturer, type, specification number and speed index may be used on a vehicle."