Hi woody agreed most of the water vapour will be checked by the plasterboard/skim. Hotter air indeed can, but not necesarily will, be holding more water, hence why condensation happens when it cools below its dew point. However cooler air in the loft being heated indirectly by the upside of the ceiling won't have had any way to gain moisture, so the water content will be the same, thus the relative humidity will be lower. Therefore when it hits the cold surfaces, it will go back to its original temperature and relative humidity with the same absolute water content which should still be above the dew point.
Furthermore, due to crap insulation, the cold surfaces will actually be a bit warmer, so there's a bit of additional protection.
That's why adding more insulation but without making sure decent vcl can cause condensation where there was none before.
However none of this invalidates your good advice to the OP which is to go ahead and seal up the holes!