You can't use 255/257 gaskets on your engine. Using a thinner head gasket will cause your valves to hit your pistons since the ej25d pistons come up just past the deck of the block at tdc. You will be fine running 5psi. We have a 9.7:1 compression ratio in the ej25d which makes it more(just barely) suitable for boost then the sohc ej engines which are 10:1. What does your build look like, are you going the custom up/down pipe route or are you swapping the cross-member so that you can use the oem pipes?