That is a nice project you have there- would love to be able to help out on it personally!
Now wait a minute- from the website:
"We started off very conservative- 15 psi, <10:1 air fuel ratio, and stock timing. The first pull netted 379 WHP. We slowly increased boost and timing advance as well as lowering the WOT ICCU setting to lean it out a bit on each pull. We also switched from 2nd gear to 3rd gear pulls after a few runs because the motor was revving way too quickly. After familiarizing with everything, but starting to run out of time, we cranked the boost somewhere into the mid 20s(gauge only goes to 20). At the stomp of the pedal, WHP shot to well over 500 WHP, but inadequate ignition spark quickly became a problem. HP peaked right away and then just fell off very quickly. Since converter flash was a factor in the peak WHP number, it should be taken for what it's worth. Rick does not wish to claim any kind of record with this pull, and neither do I. Especially because the pull was terrible over-all as power fell off quick.
IMO the words "very conservative" don't apply here if the above is true. If you really have the stock timing curve in it, yet are running like 10+ more boost now than stock ever saw that is a recipe for disaster, asap. Especially if the part about adding even more timing on top of it is true. Sorry if I'm being too simplistic here, but it seems to me that it could just be that the power curve is so abrupt and wavy from detonation (abnormal combustion) and/or heavy knock spark retard...
"Inadequate ignition" is likely not the problem there... I think it may be just falling on its face from inappropriate tuning. FWIW, mine for example has the oem stock Buick ignition, including the stock coilpack, and I run a T76 turbo with 30+ psi boost on it, + up to a 175 shot of hose on top of that and it still hasn't missed a beat. Fresh plugs gapped at 0.28". Is it optimal? No. Does it work? Sure seems to. Surely your type 2 can ignite ~ 20 psi, assuming you have good plugs at a reasonable gap.
As for backpressure, I used to run a .63 4 bolt hsg on it because it spooled better at the time. The car went 133+ mph with it just rolling it off the line. A hard launch would have likely been in the 135 range. This with a 235" S1 motor at 3650 lbs race weight (no hose, etc back then, almost 10 yrs ago now). I measured the bp at 50+ psi. Swapped hsgs to a .81 a/r and could not really measure a bp difference; very crude measurement technique though, I admit. The old q trim exh wheel is just not as good as the current GT wheels. Moral of the story, yes you will have bp no matter what you put on (within reason), and no that is propbably not what is destroying your power curve, even if teh cam is slightly out opf phase. Personally I agree with Postal on the cam stuff, but OTOH turbo Buicks seem to still run quite well with probably worse cam settings in them...
Hope that helps some...
TurboTR