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.
225,000miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. CHECK your oil, these cars use it.
Engine Build - Click Here