If your necks are easy to reset, then it's really not that big an issue. But for integral neck builders like me, then it is. And I do use braces, about 3/8" tall, and notched into the upper transverse brace and X at 1/8" or so tall. My guess is that the added stiffness has a small tonal penalty, but does help structurally.
The upper bout structure has to be viewed as a whole, and yours, being based on a builder with many years of experience, should be pretty well proven good. Though that doesn't guarantee that it can't be better. Here are some of my theories from experience and pondering...
The upper legs of the X should reach significantly above the widest point of the soundhole. That's what really carries the stress in the standard X brace design. My third guitar caved in due to an excessively splayed X, resulting in the upper legs ending equal or slightly below the soundhole center. It had 1/4"x1/4" soundhole braces, which didn't help much.
There are two distinct forces being dealt with. First is the torque of the neck, and second is the compressive force in-line with the neck. A popsicle brace is actually tall and thin if you're looking down the length of the neck, so it's half way decent for that, but no good for dealing with the torque. And if you use a tall and thin upper transverse brace, then it's actually short and wide from the longitudinal perspective. Good for torque, but not compression. Though the plate itself is good for compression, so a tall/thin UTB preventing it from buckling may be all that's really needed.
The A-frame design, where you have braces notched into the headblock, passing under the UTB, and ending at the X, deals with both forces. Functioning as pillars to carry the compressive force past the soundhole, and as beams to resist the torque bending.
I like to use a big beefy headblock extension to prevent any bending whatsoever above the upper transverse brace. But if you do that, then you need a big beefy upper transverse brace as well, because none of the stress is being dispersed before reaching it. Mine is 3/4" tall and 1/2" wide, sort of wedge shaped cross section. Tall from both the torque and compression perspectives. But that still doesn't disperse the force, so then the area below the upper transverse brace needs to be stiffer, hence the soundhole braces. The end result is that the upper area retains its shape, and the whole box below the upper transverse brace pulls up over time. But that takes a lot more movement to raise the action compared to a small angular change of the headblock.
Everything changes if you use other structures like Haans' CF tubes to take a lot of the force.
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