Pump circuit is largely dependate on a quick motion of the throttle to discharge fuel to cover the lean spot on accelerating from a low speed.
When you ease into the throttle, you notice no power loss because the pump circuit isn't needed or used . But when you stomp on it the pump is working at it's hardest and since if can't supply enough fuel to cover the lean spot then you feel it as a loss of power production. Usaully this manifestes itself as a hesitation or bog for just a second or two after mashing it.
You secondary springs may also be to loose, allowing the secondary's to open to soon. Since the pump circiut is already used up and can't help, the car goes lean untill RPM comes up and booster signal is restored. (Vaccum)
With the low vacuum levels you have indicated, I can only assume you have a long cam. Long cams already produce lower then ideal vaccum levels so the problem is pronounced.
What's this mean? The engine went lean.
Change springs in the secondary's before meesing with anything else.
Secondary's rely on vacuum that's created as air moves through the carb, not when the carb is closed, producing vacuum under the carb. You are measuring from the port that is under the carb.
With long cams, vacuum is often higher when RPM's are up and engine isn't working hard because of the overlap period. The cam's getting closer to it power band.