I assume you mean 10.5 mm!
I think carbon plus 2 layers of E sounds like a lot. I have 2 suggestions:
1 - make some test strips. Thin down some scraps with a router or I can give you some bits of 3mm corecell, maybe 6" long and 1 or 1.5" wide. Put 3 layers of E on each side. This replicates your current board. Now do another strip with one carbon and 1 E. With the thin core in the test strips you can flex them by hand and go by feel to see if the carbon plus one E feels more or less stiff than your current board.
2 - Use some offcuts of your carbon to add a few strips in the mid-section, and add that the the carbon plus one E.
In general, you have more material to bend on a big board so you should need less skin strength on a wider board. Plus, I don't think you can load it as heavily as a narrow board where you are standing right at the rail.