I have a Internet friend running that on her 919. Turned it into a screaming power house she said - basically your revs will increase about 500rpm for any given top gear speed so it is a lot more dramatic when you snap the throttle. She loves it in the hills.
Me - I find the stock gearing pretty perfect to a wide variety of riding environments and styles.
You can try it out without changing anything for open road cruising - go for a ride and keep the bike out of 6th - ie use 5th as a top gear on the open road - that is what cruising will be like. Obviously each lower gear will have a similar affect and 1st will be more wheelie prone.
i bought a 15t front on ebay shortly after i got my 919.
turned the thing into a wheelie machine, it DEF makes a decent difference. i quickly switched back to the stock 16t because i could be smoother in 1st and second in the twisties... and being smooth is key.
Tho i must say is was hella fun!
Hit me up with a pm... like stated i got a 15t that has maybe 1k miles on it.