It certainly looks like a winner.  Carving a flap out of a single piece of solid wood will probably get you the worst possible stiffness/weight ratio.  The carbon fiber should stiffen things up dramatically -- I assume you use mat with random fiber directions, but if you use cloth you should run the fibers diagonally to line of flight (45 degrees if it's woven at right angles).

It'd be interesting to see someone build flaps in several different ways, then measure torsional stiffness and weight.  Covering should not be ignored!  I can see several different ways that could be investigated:

1.  Plain ol' wood.  This is probably the worst.
2.  Plain ol' wood with silk on a 45 degree bias.  Should strengthen up, but I dunno how much.
3.  Plain ol' wood, silk on 'straight' -- if silk on a 45 degree bias makes a difference from plain wood, this will be better than plain wood, worse than 45 degree.
4.  Plain ol' wood, silkspan.
(if you're thorough, you'll try different weights and or multiple layers)
5.  Plain ol' wood, carbon fiber.  Anything with significant CF should be way stiff, but fragile if you really twist it, particularly if you stick the CF on with dope.
6.  "balsa plywood" -- stick two sheets together with grain offset by 30 degrees or so, so that top and bottom grain both run mostly along the flap, but at a 15 - 30 degree angle.  Then sand it out like plain ol' wood, with the glue joint down the center.  I would expect this to be significantly stiffer than plain ol' wood.
7.  Built up -- leading edge like yours, ribs, sheeting.
7a.  Built up, diagonal -- Run the ribs diagonally to make triangle pattern, sheet.
7b.  Built up, diagonal, no sheeting.  Done right I bet this'll be stiffer than plain ol', and it'll be lighter with careful glue management.
7c.  Built up, with CF -- this is, I think, what you're proposing.
8.  Foam core.  Like yours, only with foam instead of ribs.  Probably more trouble cutting the core than it's worth.
8a.  Foam core with/without CF.

As a control you could mill a flap out of tungsten.  It'd be stiff, but I think the weight would disappoint.

If building and testing 12 different flaps weren't enough, there's going to be a distinct tradeoff between flap thickness and torsional stiffness.  Who wants to make 24, 36, 48 flaps?

Hope I can get by with this.  Go over to the stuka stunt works site and do a search on Al Rabe's construction of his various planes.  He has quite a bit about stiffening up the flaps/elevators.   Good idea tho Phil.

Has anybody else tried this type of flap construction?  The leading edge was backed with two layers of 5gr. carbonfiber matte and epoxy, and then carved to shape.  Built the flap up flat on the board, using 1/16 in. sheet surfaces and ribs, with a ply root rib slotted for the horn.  Shaped the leading edge and sanded it down to the skins, rounding the corners a bit to avoid later sanding through the carbon fiber.  Then wrapped a couple layers of matte around the leading edge and covered the flat surfaces with one layer.

The end result is a flap with a pretty stiff torque tube at the leading edge with the carbon fiber boxing in the leading edge, supplemented by a sorta torque tube formed by the surfaces of the flap.

It's a bit more work than carving from a solid sheet, but seems to be lighter and and stiffer.  All the wood on the inside of a flap doesn't do much for stiffness anyway.

I like this method but, the downside is that after you build in this nice torque tube you have to notch away about half of it at each of the hinge locations.
I would think that in the end you will retain less than half of the added stiffness this method built in..