Shaped base wads were used before plastic shells were invented for the same reasons.

A shell is a heat engine, and anything that contains heat and keeps it from escaping aids performance.

Looking at Federal data vs. Winchester data for common loads in the Alliant loading manual shows that using Red Dot for example with the same primer and the 'correct' wad for the hull the difference is about one grain.

"The price of good shotgunnery is constant practice" - Fred Kimble