I don't think in practice it would be much different than using 12ga gauge mates in a 10ga gun, 20ga gauge mates in 12ga etc so there should be some experience with performance on the board with this.
Surprisingly ATA rules govern the chamber, no larger than 12ga, and not the bore, so way overbored barrels are legal. See page 49.
http://www.shootata.com/pdfs/2011_rulebook_4web.pdf