I asked this same question of Kirby Allen several years ago. He said by the time he finished all the work required to 'blueprint' a stock Rem 700 action, I would be within a couple hundred dollars of a custom action. Both would then do a great job. However, when it came time to sell the rifle, the Rem 700 action would not bring near the resale value that the custom action would. Reason being that there would be little in the way of documentation that the work was actually done (other than original receipt), or that it was done correctly (competent gun smith known to future buyer). In the end, I went for a custom action.
Story has a funny (not really) ending. I went with a Lawton action as part of a group buy on another site. Unfortunately, it was during the time when the company was changing hands. When Kirby got it, he ended up doing quite a bit of work on it anyway just to bring it up to standard. So you just never know......