I really hope you don't build boats for a living. The secondary bonding characteristics of epoxy is much stronger than that of polyester or vinylester resin. That is a major reason why epoxy is generally used for repairs.
Yes, epoxy-based GRP and Polyester/vinylester-based GRP will flex and expand at slightly different rates under the same stress and temperatures, but that generally isn't much of a problem unless you've made one side of a boat in epoxy-glass laminate and the other in polyester-glass laminate.
When you're talking about relatively small patches, like filling in a 4" instrument hole or 2" through-hull hole, the differences in flex and such are not really a major consideration IMHO—especially given epoxy's stronger secondary bonding characteristics and greater tensile strength.
what haffiman is saying is, that rock hard epoxy doesnt flex the same as the poly,,,there lies ya problem and where ya cracks are going to be. so to keep that from happening you need to use what everything around the patch, is made from poly,,,,, and the mechanical bond is bout the same for poly as epoxy,,, its just that the epoxy is harder,,it doesnt mean it "sticks" any better
As for your earilier post about making the hole non-circular—you don't seem to realize that by expanding the scope of the hole more than necessary to grind out the non-circular shape you're actually damaging the existing laminate and weakening it, and effectively exacerbating any problems you'd have due to the strength and expansion differences between the patching epoxy and existing polyester resin.
BTW, the fibers in any laminate provide the greatest strength when they are fully continuous. This is why chopper gun fiberglass layup is so much weaker than using tri-axial roving—since the strands are much shorter in chopper gun fiberglass. By sanding an irregular outline around the hole, you're effectively weakening the laminate all around the hole...and since the outline is irregular, it is unlikely that you can repair that area with contiguous strands of glass in the patch. This is also one major reaons I generally like to put the largest piece of glass down first, rather than the smallest—when you're sanding the surface fair, you tend to go through the outermost layers quite a bit...and that would leave you with the shorter layers intact, but break up much of the longest fibers in the repair.
You can't have it both ways. Either you should minimize the use of epoxy, due to the difference in bonding and expansion characteristics, or you should maximize the contact area because having an irregularly shaped patch will increase the strength of the repair...
Again, I really hope you don't build boats for a living or that people don't use your boats for anything serious, given what you're saying on this forum.