A few people have asked about this in the past and I think there has been a bit of misinformation (some may have come from me ).

I finally got my ISUZU WS manual back in my hands, and it says:

1988 and Earlier engines were fitted with an:
IHI RHB-6 (with wastegate). The wastegate opening pressure (boost pressure) was: 680mmHg = 13.15 psi = 0.91 Bar

1989 and Later engines were fitted with a Garrett T-25 with NO WASTEGATE. Maybe this is where the erroneous 5psi boost figure came from, but I don't see why these would run lower boost than the earlier engines.