Or maybe no one has back to back tested them on the dyno and thus no one actually knows the answer?

The size and shape difference of the stock stuffer over the HRC one is pretty small, it and the HRC reed block itself have both got to be better than stock ('21, you'd think they'd be better than '18 too) but you're almost certainly not going to be talking about an amount you can feel or even see on a normal dyno.
There are also slight mods to the HRC reed block metioned in the RS manual that aren't in the F3 manual.

Some bright spark will probably now post back that they fitted the HRC reed block, reeds and stuffers and found a horse and the motor felt loads better. Yeah, that's mainly as they replaced their 15 y/o reeds with new ones.

So, my opinion* would be "wasting your time" unless of course you're replacing these parts due to wear and tear (which is nice, carbs pop into fresh manifold rubbers very easily and reeds do wear) in which case go head (though I'd use the '28 manifold and ditch the boost bottle too) but any improvement you find is going to be 90% from fresh reeds.

*guess, maybe the '21 reedblock and HRC stuffer is worth several bhp over the stock mc18 one, I've not tried it but I doubt it very much.
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