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.