Hmmm, interesting debate. I am in New Zealand. We see a clear difference between production - a manufactured product and what you call custom - built by a boatbuilder. There is a reason they are cheaper. The boats that do best in round the buoys club racing here are NOT production. The production products lose chainplates, keels, interior liners, rudders, genoa tracks......and the list goes on. And they don't win races. This type of racing here is in any style of boat, mostly cruisers being raced and many of these boats will do regular ocean passages.
Funny, I had the impression that last year a First 40 had won the Sydney-Hobart and this year again a First 40 and a First 45 had won their classes. I also had the impression that the Sydney-Hobart was a tough race and that this two last years were not among the easiest