To me the smaller diameter tubes are only helping when using gloves, while without gloves I feel that the finger-tips become colder compared to when using a boom with a larger diameter. I will get "white-fingers" earlier on the slim booms compared to the more regular diameter booms, even in fairly moderately cold weathers.
If it is really cold (around or even below freezing temperatures) then I use the Dakine cold water mittens. Above that it is either open palm mittens or gloves. Open gloves feel a little better as there is still a bit of insulation covering each finger-tip, while with mittens there is instead a water-filled pocket for the finger-tips.
I once had a repaired carbon slalom boom that didn't have a regular grip fitted over the repair. That region (front-hand position) of the boom always felt colder compared to the rest of the boom, so I don't think that you can reduce the grip thickness very much or you will loose the needed thermal insulation from the conductive carbon. On days below freezing I will get ice on the leeward side of the boom, so after a jibe the boom is very slippery at first, until the ice is melted away under my hands. So thermal insulation is also an important property of the boom-grip.
