The "clean break" area on my Stainless Marine bracket and the swoop are the precise areas where Hermco fills the gap.(Big Shrimpin's brackets too) By closing in a little more of that void on the underside of the bracket, they add floatation.
I don't think they go so far as to hurt the performance. Water rises as it leaves the rear of the hull but I watch where the water hits the lower and I know there is more room to add some additional floatation below the waterline. I was thinking of starting at 1.5" from the bottom at the transom with a swoop that winds up approx 2 to 3" higher than where I have noted the water hitting the lower unit. (Now there is 6" to 7" from the bottom of the bracket at the rear to the waterline on the lower) If it was 4'wide that would be a couple extra square feet of floatation at rest and yet completely out of the water on plane AND being 4' wide the boat would certainly be more resistant to transom sinkage when there are four people standing at the transom. LOL.. It would need to be light enough to reap the benefits yet strong enough to withstand the beating of a barely used 300XS Merc.
Umm , did I stray?