As far as I know, it has something to do with:
-weight
-suspension setup (GT2/GT3 share the same engine block but don't have a common suspension with the Turbo).

As far as I heard, the GT3 will have the same recommended tire pressure as the GT2.
The recommended tire pressure is for mixed usage (mainly street) only and shouldn't be lowered for that purpose. On the contrary, lowering the tire pressure for street driving can actually put you in trouble.
For the track, things look different. The tire pressure has to be lowered because it raises again as soon as the tires are very very hot (track racing). So even if people lower the tire pressure to 2.1 bar, they might end up again with 2.5 or even 2.6 after some hot rounds on the track. Many people (I'm sure you're not one of them, Stephen) think that generally lowering the tire pressure does the trick (no matter if they drive on the track or street) but this is BS.

Highly recommended: having two wheel sets, one for the street and one for the track (semi slicks recommended).
Swapping 18'' tires on the same rims is not approved by Porsche because the tires can be damaged invisibly to the human eye and one might end up with a blown tire.