I'm good on meds today!
You are correct. As pan vacuum builds, the seals and gaskets have to seal or you can't build vacuum. Some belt drive vacuum pumps can pull 15-18 inches of vacuum and a good dry sump with a well sealed system can make over 20 inches.
Much over that, it gets hard to oil the wrist pins and keep stuff sealed up.
The longer the stoke, the bigger the bore the more displacement you have! Just like the are above the piston, you have that much displacement below the piston. It's like a hurricane down there. So, a negative pressure in the crank case makes oil return easier, along with crank scrapers. And...the rings will seal better. So....you can use thinner rings. Which causes less drag. And with less drag you get more power. Especially on the exhaust and intake strokes. Plus...once you get thinner than 1/16 inch rings (1.5 mm for you metric types) you can use gas ports. Which allows even thinner rings, and they need less back clearance, so they have less radial tension which saves more power. And so it goes
