I cant see how the operating temp of that little 650 is going to be much affected by the weather. I just keep the coolant above the line and it seems all good, never seen the bars go above 4, and that only when I'm sitting in traffic. I don't really tax it too hard though, Miles82 has been riding with his girlfriend on back all weekend and his seems to be running fine as well. For sure it's going to feel like a sauna when you're sitting in traffic, but haven't noticed any performance issue because of it.

That said, I'm a complete noob and probably wouldn't be able to tell the difference anyway, so take that with a grain of salt ;-)