My 2012 C5 was built in 2011 with 18" wheels front and rear, perhaps the last of the Classic 500s built that way, and also has the earlier "non-offset" front fork. When I purchased it, it was fitted with a 20 tooth front sprocket. I assume that the first owner did this. Until I counted the teeth, I had no idea the bike wasn't running the stock ratio, the bike feels fine. When I occasionally feel the need to downshift to better accomodate conditions, I do it. For reference, it is quite flat here in Chicago. In my urban, surface street riding, I seldom use 5th gear. 4th can take me up to 20 mph over posted limits, very nicely.
On the few occasions I've taken the C5 out on the expressway, the machine cruised comfortably at ~69 mph and the highest speed I've been able to reach so far has been ~75 mph. Above 70, the steering gets a bit shakey, which I attribute to the reduced trail of the early fork. I expect that the higher 20/38 ratio works against achieving the higher, ~80 mph top speed, that other riders have reported, but I've never had difficulty with insufficient torque, and I'm over 200lbs and not much inclined to tuck at my age. The bike pulls without stumbling or feathering the clutch in 1rst and has never shown any lack of power other than the top speed, air drag limit. One of these days I'll try the 18 tooth I've acquired, but I I'll probably stick with the 20 tooth.