I can't figure this out. I did a long drive in my turbocharged S2000 and on the way back I noticed this vibration. When I shift the vibration through the stick is nearly unbearable. To the point of being painful to hold on to. Upon inspection there was exhaust up the side of the motor. And certainly you can smell exhaust coming through the heating system into the cabin.

I think it's a broken weld on my downpipe. The car seams to hold my 5psi. Would a broken exhaust component give me that kind of vibration though?

 

well, possibly.

The engine has mucho torque when at boost. You know this, so Assuming the stability of the exhaust is compromised it could.

My guess would be something else....but I haven't experienced your particular situation, nor one like it.

Guesses other than exhaust:

Motor Mount (specifically Pass. Side)
Drive Shaft
Diff
Wheel Bearings.