How important are tail cutouts .?

So I just did some quick calculations....On my Weapon 73 Slalom board I measured the reduction in surface area of my cutouts assuming a stagnation point 700mm from the tail at high speed . thats just behind the front foot so probably close to reality. The cutouts actually remove 8% of the wetted area compared to not having them. If we agree with the graph above and say that at high speed the induced drag and friction drag are equal then the cutouts reduce overall drag by 4%. I will take that.
Of coarse you also have fin drag so the percentage gain of the cutouts reduces in percentage of total drag when you factor in that. netherthe less I will take any drag reduction I can get...why wouldn,t you if high performance is your goal?

FWIW, here is my experience with smallish isonic Slalom Boards. I had kept my favourite (2009?) Isonic 87 for the longest time, despite trying later models a few years younger on numerous times. It did everything I wanted it to do very well. That board has no cutouts, but it does have a slightly sliced off piece each side of the tail which turn it into more of a diamond tail than a rounded pin tail.

I even bought a hardly used identical model last year when I found one for sale as the old one was strting to look a bit tatty.

I had watched Dr Cam set his smoking PB 1hr at Albany on a '15 or '16 Isonic 80 with the very defined cutaways a couple of years ago and I really liked the look of how it sat on the water and how fast he made it go, so had been on the lookout for one for a while.

Then I saw a similar shaped 2017 Isonc 80 for sale, which is not a common thing to find, so I bought it hoping it would be better given the 8 years of extra development. This board is shorter and wider and has quite significant cut outs in the tail. It turns out to be perfectly balanced for me with my 5.7m Cambered speed sail in 20 to high 20's winds. I was amazed at just how much faster this board is than my old one for me. It seems to 'fly' a lot higher on the water reaching and downwind, and yet I can drive it upwind at 20-22 knots faster than my old board as well. On the old board I rerely got over around 37-38 knots on the speed course, although once by accident (caught in the speed run when a 30kts+ squall hit me) I peaked at 42 knots!! That was the exception. On the new board I was immediately topping 37's in much less wind (flat water on Lake George) and comfortably topped 40 knots a number of times in much less wind (25-27 knots maybe) than on the old board.

The board is a lot different shape, but the cutouts combined with the different shorter rocker line, do seem to allow it to fly higher and free'er and faster on all points of sail. Needless to say I am rapt in it.