Thore, Don't bother wit ha bypass, you loose static thrust and only gain a little top end. It also makes the install a lot tighter. I was going to go that way but after fitting in the bypass I changed my mind.

There is enough airflow out the back around the pipe that the inside pressure should be fine.

Disclaimer - I have not flown my yet since I just got the Turbine back from service and I am still waiting on mounts so I am only working off what I have seen with other F-18's as far as pressure in flight goes.