I think I can answer this myself now. Yes, newer ProAcs have better highs than the 2.5 or One SC, for example. I refer to the very high frequencies, and their superior quality in newer ProAcs, in my opinion, make the newer ProAcs sound much better than older ProAcs. They simply have more transparency, resolution, and better air, sound staging, and imaging. I recently listened to ProAc One SCs sitting next to the new Tablette Anniversaries. The latter is just an order of magnitude better in terms of clarity, instrument separation, transparency. The imaging was much superior to One SCs. The bass is tighter and more extended, but still as strong in the mix as with One SCs. The Tablettes lose nothing to the One SCs in terms of warmth, presence, or realism, in my opinion.
I listened also to 2.5s. I feel that new floorstanders are, again, just much better speakers, and that the D40R is a universe apart in quality from non-ribbon ProAcs.
I plan to trade my One SCs for Tablette Anniversaries.
While I'd love to have 2.5s at the prices they are going for now, I would far rather have a D40R, D28, D18, or even Tablettes. The sound is just much, much better if you care at all about transparency.
Having listened in the last few weeks to B&W, Merlin, Sonus Faber, GoldenEar, JMRs, Maggies, Totem, Gallo, Vienna Acoustics, Linn, Martin Logan, Paradigm, PSB, Sinclair, nothing does it for me like ProAc. It's not the "hifi" qualities, it's what ProAc does that's not captured by those parameters. They ROCK. So often I see people saying they are "musical" or "involving" or "engaging." Sure are, they kick ass and sound fantastic.
Only one other speaker might give me pause: Harbeth Compact 7s, for jazz, but not for rock.