If you are really interested in this subject, here's a TB thread from 2016 all about the stiffness of necks, reinforcement with metal or carbon fiber, truss rods, etc. A lot of good info in a 2 page thread:
Neck reinforcement rods vs truss rod
Another thread with some info about truss rods combined with reinforcement:
Aluminum reinforcement rods? Maybe?
A post from another thread about reinforcing necks with metal plates:
If you want to stiffen the neck, what matters is the depth (the vertical height) of the carbon fiber strip. The width means very little to the stiffness. Extra width just means adding more plastic to the neck.
The ideal ultimate carbon fiber stiffener bar/plate would be 1/16" wide x 5/8" deep at the heel, tapering to 3/8" deep at the 1st fret. Saw them out of some 1/16" carbon fiber laminate sheet or strip stock. That will make a super stiff neck. One of those on either side of the truss rod, and you'll barely be able to move the neck with the truss rod. You install these stiffening plates by routing a slot, say, 1/4" wide, and then putting in the plate with a 3/16" thick slice of maple beside it to fill the gap. Or, you can put a single slot down the center which holds the truss rod with a plate on either side, and a maple filler strip under the truss rod.
Also, you don't have to use carbon fiber for these stiffening plates. Aluminum works just as well. Standard 6061 aluminum plate, 0.063" thick, is easy to cut and work with. And it's actually lighter than the comparable carbon fiber bar. And less expensive
I make a special version of my truss rods that is a wedge-shaped cartridge, made for simple installation by my clients. Last year, I made a couple of prototypes of this cartridge truss rod, with a pair of these aluminum stiffener plates built in, on either side of the rod. They were like I've described above, 1/16" wide x 5/8" at the heel, tapering to 3/8" at the 1st. My client (another pro Luthier) installed one in a sample guitar neck to test. The neck was so stiff that the truss rod could barely move it 1/16". Too stiff, actually. On the production units, I'm now cutting additional slots in the outboard ends of the stiffener plates, to allow the outboard end of the neck to flex a bit more.
Using this design of stiffening plates, either carbon fiber or aluminum, you can make the neck as stiff as you want.
Remember, stiffness and stability are two different things. A stiff neck can still be unstable if it isn't built correctly. You still need to dry the wood, select for grain/ring orientation, eliminate pre-stresses, seal it up, etc. All the usual good technique.
