I bought another FS-40S to use for spare parts in case I ever need to rebuild my engine. (yes, I know some parts are different in the FS-40S-CX, but the parts most likely to wear-out are the same.) While I have the second engine apart to double-check that it is actually brand-new as advertised, I was studying the crankshaft and camshaft, and I had an idea for how to easily advance the valve timing for better operation at high RPMs. When the crankshaft is pulled backwards, as if being removed, the camshaft rotates forwards, which is the same direction it rotates when the engine is running. So if the crankshaft is held in a slightly rearward position, that will cause the camshaft to be rotated slightly forward relative to its normal position, resulting in advanced valve timing. This could be achieved by installing shims between the crankshaft and the crank bearings. Only small adjustments would be possible without moving the connecting rod rearwards enough to cause uneven loading on the upper conrod bushing and the wrist pin, but even small adjustments to valve timing can be beneficial. I made some measurements and did some math, and here's what I found:

The average diameter of the cam gear is 12.47mm. (OD is 13.57mm and ID is 12.47mm.) So the average circumference of the cam gear is 39.1757mm, and each degree of the cam gear's average circumference has a width of 0.1088mm. Installing a 0.2mm shim behind the each of the crank bearings would move the crankshaft rearwards by 0.2mm, which would conversely rotate the cam gear forwards by 0.2mm. That rotation translates to a 1.84° advance in camshaft timing; due to the camshaft's 2:1 gear ratio with the crankshaft, that would result in a 3.68° advance in valve timing with respect to crankshaft rotation.

Unfortunately there's no way to adjust the intake and exhaust valve timings separately without regrinding the camshaft (and if you have the tools to do that, then you could probably just make your own custom camshaft), but even being able to advance both valve timings together should make high RPMs easier to achieve. It's not like the FS-40S can't afford to spare a little of its peak torque in exchange for a wider powerband.