Don’t forget that for GeoGrafting to work the parts have to line up in their zero-pose, so the horse part of a GeoGraft centaur would need to be largely under the floor. The hierarchy would also be odd as the horse figure would be a child of Genesis. It wouldn’t work on a separate horse figure as, as I recall, only the parent figure gets parts hidden - and of course a horse wouldn’t be modelled to match the Genesis vertex positions.
Yes, on the last point, you would have to reduce the poly-count on the horse’s neck to match a suitable ring of polys around the Genesis waist in order to achieve a graft. This means it could not be done with a morph of the horse, and therefore, I presume, the horse would have to be re-rigged, not to mention re-mapped, if I am not mistaken. Then of course you have the texturing issue.
Hemi426 does it the old-fashioned way, and, I think, with excellent results. As a professional product I should think a standalone figure would still be the way to go (as has already been done, of course), although I seem to recall that KempSparky was experimenting with a Geo-Grafted centaur, but he modelled the horse body himself (ran into problems, I think, which was a shame because it was looking great).