I think it happens more than some custom junkies would like to believe, look at the tailoring on Loki, The Mandarin, Agent Coulson, Jack Sparrow, DX11 and so on. Hot toys tailoring is pretty damn impressive if you ask me. Not to say they haven't been out done from time to time by the best of the best custom tailors, but not on a regular basis.
Take for instance rainmans taxidriver, ok figure with a dragon level head sculpt and that jacket is mediocre at best, other pieces of the outfit are better but the some of all the parts makes for an average figure with an outrageous price.
Take again the joker outfit made by kado, amazing work but 700 bucks for a 1/6 jacket is mind blowing considering you could have bought the DX11 with kado's outfit and a buttload of stuff it comes with twice. I get the that a custom tailors time factors into the price but pound for pound value and quality hot toys wins everytime.
I have limited experience with customs, but I have to say - I was not impressed with my Rainman Marty. There was several issues I had with the tailoring. I fully expect a Hot Toys version to be superior, which is why I sold my custom.