Overfilling the pan will just increase the windage and the oil will be frothy. This will definitely decrease your oil pressure.
Overfilling by just one quart will not actually overfill the pan if the engine has poor oil return. This is an easy way to quickly find out if pan volume is less than enough.
As for oil weights, I don't think a 30 weight is a cause for concern. Most times a 50 weight will appear to provide better oil pressure simply because of the gauge's location between the oil pump and main gallery (BBC), or on the same oil feed gallery from pump to main (SBC). The gauge sees the pressure created by the pump trying to force a thicker oil through the engine, yet less pressure arrives at the far end.
A lighter oil shows as less pressure on the gauge because it is of less restriction and flows easier to the far end with less effort.
But at the same time it's true for the lighter oil to bleed off faster requiring more oil to be moved through the engine, or a slight drop in pressure.
So I am not for or against 30 vs 50 weight oil, just something to keep in mind that the factory oil pressure port's location is not optimum.
A better place for your gauge's sender is the front of the main gallery. This will provide a more relevant reading of mains pressure in a worst-case-scenario. In other words, I don't care how high my oil pressure is, I care about how low it is.
Maybe you're overheating your oil. You say your pressure drops after WOT. Got a pan gauge or IR gun?