performer RPM air gap v.s ported LD4B

The ports of the LD4B are smaller than the LD340 or the Performer. Gasket matching at the intake/head interface will not change the restriction - so I would say yes, you will see/feel a noticeable improvement with your combo.

I had the LD340 on a fairly stout 340 and changed to an Airgap and found no appreciable difference. In your case I believe you will. You might also think about going with the RPM Performer if you do much driving in cooler weather as the Airgap takes ±10 minutes to warm up enough to let some heat to the carb.