It really depands..
I will go for metal, if my remember well that 2 mm will bring down to 8.7:1 ( or 8.5 pls correct me..)
after that, can run higher boost. Because I am in process of upgrade my turbo to T04E with external wastegate and HKS ex manifold, 550cc injectors and R34GTT fuel pump (don't ask me why, I will explain later....a fxxk long story)
..and I was told not to run higher then 1.2bar before I change my head gasket, and at 1.5bar I should consider forgy piston, rod and possible crank balance...however I will try to see how the T04E can do then slowly up the boost..
oh...metal can survied better abuse from high boost...