How are you testing for boost leak?
Here is how I tested
https://www.youtube.com/watch?v=-7ZPmLl_7xI
Are you saying the block off plate on the manifold was leaking? How did you test that?
0 PSI at WOT is actually not necessarily a problem with a centrifugal SC, it depends on at what RPM this is happeninig. Boost will = 0 at WOT below a certain RPM.
Does your boost gauge EVER see boost? Seb sees 4.5 which is coming from the MAP sensor in the manifold. Your gauge is being fed from a vacuum line coming off the manifold. If Seb see's boost, but your gauge never does, that might be a pinched/torn vacuum line.
Not sure why he only sees 4.5 psi though. Maybe your manifold not sealed, but not sure how you tested that it's OK.