Maybe some of the vets can chime in whether or not this could actually be related to the injectors.

I mean, you have gone through pretty much everything in the drivetrain, and I just can't see the drivetrain causing the hesitation you are talking about.

If your pressure is good, I would say all your connections are up to snuff as well as the passages...the only thing left is maybe some gunk in the injectors that is only apparent when they try to sustain.

Considering everything else you have went about and try, it might be worth the 15mins to swap the 450s back in and see what happens. I would assume you have an old link tune that you could load up to match the injectors?

edit: if it does turn out to be the injectors, at least you have Ziggy to stand behind his products...depending on how old they are of course (can only be considered responsible after a certain point)