16/40 is standard on your bike, i would just put a 15/40 on and see how you go, if you want more punch then pop a 42 or 43 on the rear, plenty of punch for road use, standard 106 link chain as well:thumbup
I found standard gearing way too high for me, I would go 15:42 best balance of easy to ride in 30mph zones and v. slow bends plus outstanding out of bends etc. Have moved on to 15:43 recently (theory - "if a little of something's good then more of it must be better") and there's not a whole lot of difference in riding but i figure its even thirstier and so would probably go back to 15:42 at some point.