Sound boxes and small entrances won't let draughts in, my varroa floors have the slot over the board filled and are on enclosed hive stands, queen excluders - with some wax between the wires - will reduce convection above the cluster (and that's not so different to a wild colony with unoccupied unused stores up above them anyway) and it's the way I usually do it, although I admit maybe a week or two earlier this year.
I have considered putting a 9 or 10" square of flexible plastic over the top of a colony when putting extra supers on, so they only go up round the sides of it when they are ready - a semi-commercial man near here used them over the brood chamber instead of queen excluders. This would stop warm air convecting up away from the cluster if I'd used them instead.
I go with the weather... I believe east Scotland was warmest today, this evening the forecast shows 9 for London and Cardiff, 11 for Edinburgh, so it might still be winter with you but it's not here. I was gardening in shirt sleeves and wouldn't have done anything to the hives if it had been cold and horrible and they hadn't been flying well.
The reason I super early is to ensure that they always have enough space and never feel cramped, and I think that as a consequence I don't suffer from early swarms and it might also help explain why I usually get a belting honey crop - a good proportion of it in the spring.
Yes, I'm also a 3-hive owner now, but with 4 nucs on standby... but at some time in my 35 years with bees I've been up to 12 hives.
