Was at the shop the other day and was discussing this with the tech since when I asked about the 250 spine shafts being lighter he was thrown off.
I ended up buying three 250 spine shafts to play with and so far have had great results. I was previously shooting 300 spine FMJs, but am looking to pick up FOC. I was able to go from the standard HIT insert to the 50 grain brass insert, which combined with the lower gpi led to a net gain of just 19 grains, but resulted in FOC increasing 3%.
Minimal trajectory difference out to 40 yards, haven't shot further yet. Will probably increase my draw weight a few pounds and to gain that speed back.