I am thinking that I would shorten that distance a fair amount Kirk. Absolutely no scientific basis, but I shoot both 5.5" high back shields and also 4" shields as well as cut 4" LP banana's.
I have often said, you can shoot almost any arrow if it has 5.5" feathers. Man do they catch the wind. But if there is a breeze, they catch that too. And they are noisier, both in the air as well as in the quiver. (that said, they must not be TOO noisy, lots of critters were killed by arrows bearing those style feathers)
For squirrel rounds, I often place 4- 5.5" feathers on the shaft, which I think slows it more, but not like a full fly flu. Remember, you tend to measure FPS at or near the bow, and that won't show you much since they don't slow the arrow down till after the arrow is on its way.
I think the real answer, for your gear and style, is to make one of each and shoot them side by side. Although TradGang gives us immediate feedback on a lot of topics, nothing beats having done it yourself for your own experience.
I am thinking that at 40 yards, you will see a fair difference. At 15 - 20, maybe not so much.
CHuckC