I have never had to grow ficus indoors so I don't have the experience. we repot in summer because that's when the trees are actively growing here and they react better to root pruning. I guess that as long as the tree is actively growing it should react well to root pruning even if it still indoors.
I have noticed a definite slow in growth when ficus are moved from one place to another. I guess it takes a few weeks for the leaves to adjust to changed light levels, etc. I would avoid repotting immediately before or after moving.
Maybe some ficus growers who do keep trees indoors for winter would like to make suggestions?
I grow this (below) ficus indoors in winter and outdoors in summer. This pic is from the fall - I just moved it outdoors for summer.
Ficus Microcarpa is not a "dramatic" variety - they don't tend to drop their leaves even with major changes in environment or repotting. I'll even keep mine outside near freezing temps. I don't use any special conditions in winter - just a north-facing bay-window. I agree that generally repotting should be done in summer when there is vigorous growth, but if your getting better growth under the MH light, then I wouldn't think that would matter much. I've done winter repots and pruning, mostly because I made a mistake that I had to try to fix when I was very green to bonsai. I wouldn't suggest it, but the tree also bounced back when I made that error.
I like the idea of a drench and making sure it is watered enough. Under those lights with coarse soil, especially after a repot, it might need daily water without fertilizer.