Just thought I'd stop in and report my experience.
Gun: Gen 3 G20SF (2012), stock barrel, stock recoil assembly.
Ammo: Federal Champion 40 S&W, PMC 200 gr. 10MM.
Was shooting at a ~8" bullseye target at 10 yards. All rounds chambered and fired without any problems. However, the accuracy seems to have suffered significantly. I fired 10 rounds of 10mm, all rounds grouped ~3". I then shot 10 rounds of the 40 S&W, all rounds hit ~4" lower than the 10MM rounds and had an ~8" group. Thinking that I was probably just nervous, I put out a new target and tried again. 10 rounds 10MM, followed by 10 rounds 40 S&W. The results were similar, I didn't see any keyholing but the grouping was definitely low and loose.
Nice to know that the 40 S&W will work in a pinch with the stock barrel, but I'm not going to make a regular practice out of it.