My '02 definitely pulls harder with cooler air, but I have noticed that with cars too.
It makes sense, cooler air is denser air, so more power. I think we just notice it more on the Enfield because they produce such modest power to begin with that even 1 hp is a substantial gain.
Otherwise, once the weather gets consistently in the 40's I would switch to a lighter weight oil, say 10-40 rather than 20-50.
I never need the choke until the weather is below 50 and even then only for a couple blocks.