I would think that if you have a leak at the up pipe, you would clearly hear that.
Have you removed the mufflers and taken the car for a ride ?
IMO that's easier then messing with the up pipe gasket.
175,000+ miles, 5/20/12 new ej257 short block, 8/20/11 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. Also CHECK YOUR OIL, these cars use it.