You can probably rule out alot of things yourself if you think about it logicly.
Realisticly a leak from a pipe, intercooler, d/v, inlet isnt going to manifest suddenly at 5k, as you say the circuit is presurised well before this point with no issue. With that in mind you can probably rule out pipe collapse intake side. Its failry unlikly that having small exhaust(2.5 is really too small) is going to cause that kind of issue either.

What i would do to rule out electronic issue is bypass elec. boost control and run it up again. If its still pulling back you know its nothing to do with the ecu as it'll no longer have control.