Anyone else know of any other downsides to using the 45S over my cold light head and below lens filters?
I can't think of anything major. You might have to make your own printing Y/M table (though the Ilford provided tables for Durst and Omega work fine for me, I would hope the info provided for the Beseler head was just as good).
You can never go wrong getting more darkroom supplies. I have multiple 'spare' 4x5 and 8x10 heads and power supplies.