My pair of pennies:
The k bikes are warm. They'll keep you a little warmer in moderate winter, but they are still motorcycles and you are still exposed to the wind. You might get a little benefit from a warmer tank after the bike warms up a while, and you might get some slightly warmer legs and crotch.
I wouldnt really count on these bikes to keep you especially warm in winter, not without the stuff you'd want to have w any winter riding. Add heated grips and wear winter gear, and it might be slightly better than another bike that doesnt run as warm.
If you want to stay warm, dont line the bottom of the tank with foil, it will be detrimental to your goal. The radiator heat will warm the tank slightly, which can help warm you a little.
You WILL DEFINITELY notice these bikes running warm in the summer. This is when you might appreciate that foil tank lining. It will be much less obvious in winter.