I would not do it that way for a number of reasons but it's up to you. It takes me all of about five minutes per thru-hull to get them out with a pipe wrench. New thru-hulls are cheap even if you destroy the old ones..
Curious, what are the reasons?
The reason I'd like to do it the way I'm planning is that the through hulls are installed with 5200, so getting them out will not be without heartache. Plus, if I pull the through hulls I'm sure I'll be doing something that will have an impact on the warranty, or at least the builder certainly will claim that in the future if there is a problem, whereas merely adding flanged adaptors cannot possibly give rise to such an argument. (And Dawg, because I'm doing this over the winter, I will have sailed the boat a bit by then, and believe me I'll be checking the through hulls, so if they were not bedded properly in the first place presumably I'll learn that this season. In any event, as noted, if they're not bedded properly now, I have recourse; if I pull them and then have a problem . . . ) Plus, I actually don't plan to through bolt the flange to the hull. I know, I know, but I just don't like the idea of increasing the number of below-the-waterline holes in the hull by a factor of three. I'm planning to 5200 the backing plate to the hull, then 5200 the flange to the backing plate, and screw the flange to the backing plate (with screws deep enough to get bite, but not deep enough to go through the backing plate to the hull). Assuming I can cut the through hulls to size with them already installed, this should not present a problem, unless I'm missing something.