The shear forces will be on the studs holding the adaptor to the hub, not the adapter itself (it is just a solid lump of metal afterall). With the higher offset of the porsche wheels the shear forces on those studs, will be no more than those applied to the studs that would hold any other wheel directly to your hub.
As long as the studs are grade 8.8.. your drive shafts are more likely to fail under shear before they will. Also, you don't need as many turns as you may think to get a 'solid' fixing so the adapter doesn't have to be all that thick.
A countersunk 20mm bedding in a steel adapter gives you a failure load of 11 metric tonnes PER BOLT! Each stud itself can withstand a shear load of 5.8 metric tonnes.(23.2 tonnes per wheel). We were talking about this in the office, trying to think of a situation where a ~1000KG car might be put under those forces at one corner. You'd basically need to drop it off a high-rise building on to one wheel to recreate those forces.. at which point we all agreed something else would probably fail on the car first (suspension / turret most likely)
So according to a bunch of bored structural engineers, wheel adapters are pretty damn safe. I'd choose steel over aluminium though.
