how well does beesputty stand up to vulcanization, in your experience?
Unfortunately, I still haven't tried the Beesputty, despite buying it in May 2016, according to my records

You can't rush these things

I bought it from Heresy, and I remember Andy saying good things about it at the time. However, I have no idea how much he did with it in practise. My big problem with polymer clays is the cooking in the oven part, which I imagine would not end well on a plastic figure conversion.
I always dig out my greenstuff when converting, and there's never any casting involved. My few tentative dabbles in that direction went nowhere

Anyone tried Geomfix?
I'd never heard of that one until now. I bought some stuff called Cold clay (not coldplay, as Google keeps on insisting) back in 2011, and this seems a similar sort of thing. Cold Clay was discontinued a few years ago, but ended up being used by me as a Milliput substitute.
Apparently I also bought a sample of magic sculpt at the same time. I wonder if it's still usable, as I don't think I've ever opened it (I think I'm detecting a theme...)