I think it's mostly in the mind, but I like my bats to be no narrower than 104mm, and prefer it to be 106 - 108mm. Mostly because that's probably the narrowest bat I've played with in my life. Ideally you shouldn't need the outer 10mm on each edge, and would be about to middle most deliveries, but I'm not that good, so eh.
With the new size restrictions in place, there shouldn't really be any need to narrow down a bat to get it to be big and light, as it can only be so big. The only reason I can think for you wanting a narrowed down a bat other than weight issues is if you want larger than 40mm edges to meet your preference for a THICK edge and for it to still fit through the gauge.
Anyone with some knowledge of aerodynamics know if narrowing down a bat could have much effect on the aerodynamics? And if so, how much narrowing would you need for let's say, approx 10% increase in bat speed?