Thanks everyone for the responses, I'll run with 19kV. As I said I have run 25kV before without issues but maybe that extra 5kV is enough to push things over the edge.
Btw this will not be an x-ray supply, it will be 3x 6.6 kV pole pigs running on 3-phase. These can be run in either star or delta to provide primary cap voltages of 32kV or 19kV respectively (after inductive voltage doubling). I will go with delta for 19kV.
Regarding cap and primary losses. I have run two large coils at 6kVA+, one with a primary voltage of 15kVp and one at 22kVp. The 22kV coil certainly seemed to be much easier on the gap but other differences mean it isn't a true apples with apples comparison.
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