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From personal experience...peeing when camping in cold weather does allow you to stay warmer. I am no thermodynamics guy, but I am guessing that you expend some amount of energy to heat the urine in your bladder to a constant temp. As it is no warmer than your core temp, you get no heat from it...you simply have to put heat energy into it. Get rid of it and you can use your finite thermal energy to heat the innards that remain.
As for losing more heat by stepping out of the tent for a moment, the transfer of heat between your body and a liquid is somewhat more efficient than between your body and air. Air is a poor conductor of heat. Also the time in the cold -- even if your bladder is pretty darn full -- is much less than the time your are likely to be stuck warming the excess urine.
On the sleeping bag front, I'd go with wool/synthetic socks and a wool/synthetic cap...and nothing in between. The big risk with clothes in a sleeping back is that they will trap moisture against your body (and the moisture will conduct the heat away more quickly). If you wear wool or synthetics, no worries. Cotton sucks (literally) the heat away.
On the plus side, going starkers -- especially when your sleeping bag is zipped together with someone else's -- has other advantages beyond heat retention.
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