I am building moulds for a pickup truck camper, meaning my dimensions are rather big. I completed 2 out of 3 needed moulds in polyester (gelcoat + CSM + infused cored laminate) and there is a quite noticeable shrinkage. In my case, it is rather sagging than warping of the planes, but it still doesn't make it any better. 
For my third mould, I am contemplating going with epoxy, which I already have from another project and which cost me the same as locally sourced polyester.

I understand the benefits of gelcoat in polyester system (hard and polishable surface layer that can be sprayed), but do I really need it in epoxy system?
My thought process is as follows:
- even if there was an epoxy-based gelcoat, I still would hesitate to spray it for safety reasons,
- PE-based and epoxy compatible gelcoats are expensive, not easily available and with a short shelf life
- if I use a textured PU coat on the final product, then the quality of the surface finish of the parts is mainly for easier de-moulding than for cosmetics.

The plan would be to slightly thicken my infusion grade epoxy with fumed silica and apply it with a roller, then partially fillet the corners, then hand laminate with 316 g/m2 cloth, then infuse with heavy skins and 4mm core mat.

If you are planning to do a skin coat with epoxy and the 316g cloth, then why not use a vinyl ester tooling gelcoat and then a vinyl ester skin coat. Let that cure, then infuse your heavy reinforcement with the epoxy. Epoxies should bond well to vinyl ester (personally I would still key the skin coat first just to be safe), and you don't have the issue of having to hit your infusion within the bond window for your epoxy skin coat.