Well, it depends on a lot of things.
Mileage
Age
Wear
What turbo setup
What fuel setup
How good the tuning is
I'd say 8-10 psi would be semi-safe on 93 with decent tuning, althought 12 might be a goal with a really well rounded ECU tune.
Pay the money for tuning, or pay 3x as much later when your engine grenades.