its difficult to answer your questions, since youre asking the wrong ones. What do you mean how much can the head handle?

my head work reco's:

Jwt S4 cams
Greddy Intake (i cant remember if you had this or not)
1.1 mm cometic headgasket
JWT valve springs retainer package or equivilant.


the better question is, when you upgrade further, are you going to keep your boost settings where they are?