So for the record - I have managed to get exposures down to well under a minute at 8x10 without needing to open past f/5.6. What I did is I painted the inside of the mixing chamber with several coats of good quality artists acrylic in titanium white (better than zinc, as it's less transparent). I also lined the back of the bulb reflector with aluminium foil making it as tight and smooth as possible.

I'm not sure how the paint will hold up over time but is going fine so far and I have been printing a fair bit. The foil thing is not an ideal solution, as it's not fully reflective, but you can see clearly that it is reflecting at least some of the magenta light that was being transmitted through the reflector back through into the light path. One problem now is there seems to be a lot more reflected light inside the head itself than previously, bouncing out through the louvres and into my darkroom. Probably cause of the uneven parts of the foil creating irregular reflections. It's not causing huge problems with my setup I don't think, but a better solution would be to try get some reflective metallic paint and use that on the back of the reflector + maybe some black paint in key spots.

All up - the light seems a lot less yellow, and is definitely much brighter. I can't compare contrast grades with a standard to make sure it's printing at 2 unfiltered, but it seems reasonable and is a lot faster than before.