Hi Alan, maybe a bit late responding here, but I have cast a fair bit of metal: first with the old range of Prince August Napoleonic moulds, then with my own creations. You might have gathered much of this already from Youtube etc, but some suggestions:
*The mould may perform better once warmed up a bit. I used to warm my moulds by placing them around the hot plate (n.b. 'around', not 'on'!) while the metal melts. I know others warm theirs in the oven.
*Try varying the clamping pressure used to join the mould halves together. Use boards to spread the pressure evenly across the moulds.
*Different white metal alloys will have different flow properties.
*Lightly dust the inside faces of the mould with talc and knock off any excess prior to casting.
*I would avoid centrifugal casting at this scale, it may well solve your problems, but a lot of hassle for just this. I would tap the mould smartly onto the counter just after the pour/tap it with some pliers (taking care not to spill it etc).
Well I suspect the market is there, I remember helping a guy out in Canada years ago on the Hat forum, he was after wagon type wheels and I sent all I had, nothing that would have helped me had I kept them though. One of the guys on Bennos where I asked the same question suggested using resin in the same mould and showed an example, and resin may be the answer. But for me my mould making days are on hold.