The project sounds OK and certainly one thats been proven in that capacity before but apart from the shaft (which can have as much against as for like drive line back lash that can lock wheels although I guess their ABS would fix that), the engione got many more ponies? or a fatter torque curve? the Boxer could lower CoG somewhat.
I mean I'm all for people tinkering for the sake as well to have their own thing, just wondering if your shopping for a perceived benefit? If you like shaft drives and want some more torque maybe a Guzzi 1200 with shaft will be around and it will also have a lovely V profile if viewing it from front on. I saw the other day a US development of a V4 transverse engine for a bike at about 1600cc I think.
Found it, cop a gander at this, that would be PHAT as the kids say!
http://motusmotorcycles.com/kmv4.html