Have you read the BPU section in the tech articles mate? some good stuff in there.
Main thing to remember before upping the boost or going for more power is to give it a full service with decent parts and fluids and check everything is working fine. Also check the coil pack clips, these break down with age and can cause miss fires etc when upping the boost.
Also I would look at going to a colder grade spark plug when upping the boost. Grade 7 will be sufficient for BPU.
I use NGK coppers BKR7E that I change twice a year (only do about 5000 miles if that), £12 for 6, again a good post in the tech section about this.
http://www.mkivsupra.net/vbb/forumdisplay.php?64-Technical-Reference-amp-FAQs