You need to consider all the various parameters when up-sizing wheels and tires the closer to max you get the less wiggle room there will be
James setup is a great no-brainer that was used on the factory 964 Cup cars. Factory used both 17 and 18 but the wheel width and ET was the same for both
8ET52
9.5ET68 used w/ a 21mm spacer for a net ET47
235/40 and 265/35 at 25.4" & 25.2" tall are a good match for tires being close to the same height which aids ABS function and not too tall which aids acceleration.
This car(a very successful club racer) uses 993 8ET50 and 10 ET65 x18 w/ spacers in back(unfortunately I don't know the spacer size but they look to be ~21mm for a net 10ET44, Tires are 245/35@24.7" and 275/35@25.5"
others have reported using
8.5x17ET51 & 10x17ET44(or 9.5ET51)
8.5x18ET50 and 10x18ET44 w/ 245/34 & 285/30 tires
8.5 x18ET56 w/ 245/35 & 10x18ET65 w/21mm spacer 285/30