I play a lot of wide-ranging repertoire (Debussy), so a timid sound beyond the break just doesn't work. I did identify a perfectly balanced B a couple of weeks ago, but a more decisive buyer with deeper pockets bought it before I could decide
The imbalance you speak of is a classic tonal profile not only of a tired piano, but also many pianos right off the lot. The structure that a soundboard must provide in that range is significant. It is a structure that is hard to achieve at fabrication and hard to maintain over the long haul. Once the structure is gone, it's gone, and it ain't coming back with that board.
That said, the picture is more complex.
For instance, a really great piano that is out of tune, even with a wonderfully live and vibrant board, will sound weak in the high 4th octave to low 6th octave. "Out of tune" does not even need to mean sounding painfully sour, but rather "out of tune" can mean the intervals and unisons are not tight enough to "couple". You get 88 notes acting as uncooperative individuals rather than 88 notes working as a cohesive whole. As you might expect the sound is sometimes considerably weaker...not sour, but weaker and with a grating percussive attack. Couple the unisions and intervals and the treble might improve considerably...but the coupling will only last a couple of weeks after the tuning...alas...
Sounds like you like Debussy. So do I.
He, like other 20th century masters, understood that supposedly "dissonant sounds" are consonant unto lush, in their own way. This means he plays with a tone's high partials big time. To really turn this music on, you need a piano which achieves its sustained treble through "structural sustain" rather than brightened short sustain sound you refer to in your post.
Interestingly though, the instruments Debussy played had significantly less sustain than pianos you are looking at. This shorter sustain though was balanced with a less powerful bass than we are used to. The bass was able to complement rather than overwhelmed the treble. The bass contained more tone and way less "growl" or "snarl" than most modern pianos shoot for.
The power of most modern pianos, power which is often relegated to the low end of the keyboard, can make life difficult for this music. The low registers for Debussy and Impressionists sing better if they are allowed be sound more like tones rather than Late Romantic "growl". You might keep this in mind as you look for a sound, as different styles prefer slightly to largely different tonal profiles. Any piano would work in any style, but some really sing in a particular style.
My own aesthetic taste prefers instruments with plenty of shimmering sustain for Debussy. So personally I would be looking for a high treble sustain piano if this music were high on my favorites list. Pianos which favor high partials really work with this music.
To this end, Rib Crowned & Supported boards can have trebles which are habit forming. This is specifically why my I design and fabricate this style of board, instead of standard modern compression crowned boards...RC&S boards can really pull this treble sustain off beautifully, but their sound is not everyone's cup of tea. I would suggest you try out a piano with Rib Crowned & Supported belly as you try and define what you are looking for tonally. You may be very taken with the sound.