(Note:  much of this information was obtained from the 1995 and 1996 Porsche Service Bulletins and the March '98 issue of "Christophorus")

I don't know if you are aware of it, but Porsche does extensive testing on the tires that go on their cars.  This involves high speed testing at either the Nurburgring or Mugello race tracks, plus lab testing of maximum speed, wet traction, wear, and noise.  For information, when a tire is approved by Porsche, it then will receive what I characterize as an "N" stamp, such as "N0", "N1", "N2", etc.  Only tires approved by Porsche will receive these ratings.  There is no real difference between an N0 or N1 rating; this simply means that when a manufacturer makes a change to a tire and Porsche re-tests it, the new rating for that tire changes from N0 to N1, for example.

From the above sources, the following are the list of Porsche-approved tires with the "N" specification.
 

Bridgestone Expedia S-01	N0, N1, N2
Bridgestone S-02	N2 or N3
Continental Sport Contact	N0 or N1
Dunlop SP Sport 8000	N0
Goodyear Eagle GSD	N2
Michelin MXX3 Pilot SX	N0, N1, N2
Michelin Pilot Sport SX	N0
Pirelli PZero	N0 or N1
Pirelli PZero Direzionale	N2 or N3
Pirelli PZero Asimmetrico	N2 or N3
Pirelli P700-Z	N0 or N1
Toyo Proxes F1 S	N0
Yokohama A-008P	N0 or N1
Yokohama AVS S-1Z	N1

The following is a factory-original rim and tire size summary for various Porsche models:
 
 

Rim Size	Offset	Tire Size
(F = Front R = Rear)	(mm)
928GTS
7 1/2 X 17   F	65.0	225/45 X 17
9 X 17         R	55.0	255/40 X 17
968
7 X 16          F	52.3, 55.0	205/55 X 16
8 X 16          R	52.3, 52.0	225/50 X 16
7 1/2 X 17   F	65.0	225/45 X 17
8 X 17          F	52.0	225/45 X 17
9 X 17          R	55.0	255/40 X 17
9 1/2 X 17   R(1)	68.0	255/40 X 17
(1) - requires 17mm spacing ring and longer wheel studs
'89 - '94 Carrera 2 and 4, RS America, Speedster
6 X 16          F	52.3	205/55 X 16
7 X 16          F	55.0	205/55 X 16
8 X 16          R	52.3	225/50 X 16
9 X 16          R	52.3	245/45 X 16
7 X 17          F(2)	55.0	205/50 X 17
8 X 17          R	52.0	255/40 X 17
8 X 17          F(3)	52.0	225/45 X 17
9 X 17          R	55.0	255/40 X 17
9 1/2 X 17   R(1)	68.0	275/35 X 17
(1) - requires 17mm spacing ring and longer wheel studs (2) - requires new stop washers in steering rack (3) - America Speedster only
'91 and '92 911 Turbo
7 X 17       F	55.0	205/50 X 17
9 X 17       R	55.0	255/40 X 17
993, C2S, C4S, Twin Turbo
7 X 16       F	55.0	205/55 X 16
9 X 16       R	70.0	245/45 X 16
7 X 17       F	55.0	205/50 X 17
9 X 17       R	55.0	255/40 X 17
8 X 18       F(1)	52.0	225/40 X 18
10 X 18      R	65.0	265/35 X 18
10 X 18      R	40.0	285/30 X 18
(1) - Requires steering rack support bracket on '95 and '96 993. 18" wheels not approved for mounting on '95 cars before chassis 99_7_SS_323614.
Boxster
6 X 16          F	50.0	205/55 X 16
7 X 16          R	40.0	225/50 X 16
7 X 17          F	55.0	205/50 X 17
8 1/2 X 17   R	50.0	255/40 X 17
7 1/2 X 18   F	52.0	225/40 X 18
9 X 18          R	52.0	265/35 X 18
996
7 X 17          F	55.0	205/50 X 17
9 X 17          R	55.0	255/40 X 17
7 1/2 X 18   F	50.0	225/40 X 18
10 X 18        R	65.0	265/35 X 17