Man, those turbo drain gaskets are sneaky. I always replace that gasket as a matter of course now when the turbo's off, even if it looks okay. The heat up there just cooks them until they get hard and stop sealing. Glad you found it, that's a real headache spot.
The idea of tightening a clamp on a hot turbo drain gasket sounds like a perfect way to crack something. I'd rather just replace it and sleep at night.