Re the talcs, there is no way the customer will ever know whats in them, but there are many different grades and grinds with each of those grades, like the old heavy weight body filler vs the new filers, aka light weight.
With that said the better primers will have very little talc in them but may have a small amount of 2-4 different grades.
Instead of using the talc as fillers they will be resin rich and some may have nano additives in place of the talc as a filler. AKA, slang of mircopheres, be it glass or a mixture of two or three different sizes air filled beads.
The nano beads will do everything from acting as a volume filler to even in some cases, work as small roller bearings to make sanding easy, this allows you to use a mixture of better resins, that if used without the nanos would just be impossible to sand.
Back in the early 70's, Dupont had a state of the art primer, called Kolar or colar, touted as non shrinking and just state of art at that time, but you just could not sand it, that was before nano's came into play and the primer was a bust as just too far ahead for its time. Today you could make that same primer sand like butter and not compromise any of its non-shrinking properties.
Of course the other option is use a minor amount of a low grade resin and stuff the primer with a mixture of fine, medium and coarse talc and watch it soak up as it gets sun time.