As far as the carb it's really simple physics. It's getting the same amount of air passing through at 2000 rpm as a dual carb is getting at 4000 rpm. It's also why the dual carbs perform better at the higher revs, but for just cruising around town that single carb will always be the smooth choice.
The Thunderbolt was the "econo" version of the Lightning; one carb, no tach, but I have no idea if engine internals differed. I recall that the Norton Commando's transmission shift pattern was one up, three down, which was the opposite of most motorcycles. If I'd had five grand I would've bought the Thunderbolt sold in my area a couple of years ago, but of course if wishes were horses beggars would ride.
