everyone else has beat me to it, but based on the slight detectable change in note near the peak rpms, I'd put good money on either valve springs being too weak, or ignition starting to fall down.

If it was exhaust backpressure, it'd slowly fall down as rpms increased, same as for the carbs, this is more abrupt, so either the spark is breaking down, or the valve springs aren't being kept under control.

Rough and ready check - try reducing spark plug gaps down to around 25thou (imperial) -which is technically 'too narrow' for most apps, but just try and see what it does. If it doesn't improve whatsoever, it's almost certainly valve springs.

It's possible that the springs are 'right' but that in order to get the right seat (and by extension nose) spring loads right, they need spacer/shims under the bottom of them. There's really no good rough n ready way of checking. About the only thing you might try is to try (when the valves are seated) grasping the retainer/top of the spring on each in turn, and see if you can twist them around. Given the cam and it's likely rpm range, they should almost certainly have enough seat tension that you can't rotate them by hand, even when on the seat.

If they can be, bite the bullet, get the head off, and get them checked and re-installed at whatever height they need to be for the correct pre-load (which your cam supplier should be able to specify based on the cam they sold to you!)

The engine just sounds too strong for it to suddenly run out of air or have a restrictive exhaust.