I think the only G28's that get cracked windshields are the ones that have had the windshields trimmed to fit, allowing many places for cracks to start around the edge. I didnt want to trim my windshield for this reason, so I came up with the slotting of the fenders to allow the windshield to slip under and not crack. I also did it to my friends Murcie, and he has been in several front end collisions (1 major one) and the windshield has not cracked yet (even though he went under the minivan in front of him and had the vans right rear tire sitting on top of his left fender). Once the windshield is bonded down, it adds alot of rigidity to the A pillar frame structure.
I drove my car once without hte windshield in place, and everything there was racking over every bump. Once I put the windshield in and bonded it down, it became as stiff as a board (maple, maybe even oak) and has never moved since.
I think what you want to do wont hurt, but from experience I can tell you its not necessary.