I wouldn't necessicarily agree that a narrower rear 'handles' better, per se, but would agree that it feels, initially at least, easier to throw around. The 240 rear takes a bit of effort - or rather, getting used to - but once you are used to it you won't have an issue with the handling because of tyre size. More important is tyre selection.
Having ridden M109s and a Harley Rocker (both have 240 rear), it's a far better proposition on a bike as big as an R3 than those others. I found the M109 in particular a tough bike to manoeuver but not so the Rocket. The T'Bird has a 200 rear and, IMO, handles MUCH (infinitley) better than a Harley with same size (eg Fat Boy), so I guess as long as you are riding a quality machine with proper set up for the tyre size, it shouldn't matter what size that is.