I'm not sure if the series of remastered cast albums we got mostly between 1993 and 2007 (when I believe the last major batch of Sony Broadway Masterworks remasters came out) could be improved even with modern technology. Most of them WERE a huge improvement on the early CD releases of older cast albums that we got in the 80s and early 90s though and even as a teen without much money I quickly made the upgrade for things like Company and A Little Night Music, with the original CD releases simply not having great sound quality.
Of course the remastered releases often included tracks that had been missing from the earlier releases (A Little Night Music had Night Waltz II, as well as the bonus film soundtrack version of Glamorous Life) or expanded (Anyone Can Whistle had an added 2 or so minutes to the Cookie Chase, A Chorus Line had a few extra minutes to the montage) and often demos, etc.
Sticking to Sondheim, when the RCA catalogue became the property of Sony was when in 2007 they released all the pre 1990 RCA Sondheim original cast albums under Masterworks with remastered sound and bonus tracks (frankly any CD from roughly 1990 on wouldn't benefit much from remastering.) Frustratingly, only ONE Sondheim original Broadway cast album never got the remaster treatment, I assume because it was never a great seller--Pacific Overtures. And it could really benefit from it (it must do alright as the old 1980s release seemingly remains in print...)
Another cast album that really could benefit from a remaster is the American Premier Cast album of Evita--again the existing CD release sounds pretty muffled. Most of the major Andrew Lloyd Webber cast albums from Sunset Blvd and earlier got remastered editions in the past 15 or so years (Aspects of Love even added about 8 minutes overall to bits that had been cut before due to CDs at the time being able to hold about 72 minutes, not 80 minutes like now.) But that Evita recording is still owned by MCA and not the Really Useful Group so didn't get the treatment (instead they did remaster the concept album.)
Anyway, I'm a bit obsessed with all of this kinda stuff--I sometimes dread when an album gets a new remaster because I'm not sure if it's worth the upgrade. The Kritzerland OBCR Follies certainly was (and that was a proper "remix" and not just remaster.) I skipped the last Jesus Christ Superstar (concept album) anniversary set and remaster that was so expensive because the remaster from just a few years earlier, on Verve, sounds absolutely stunning to my ears and I can't imagine it being topped.
Thomas Z Shepherd, the great cast album producer, was in charge of most of the 1998 Sony remasters of Columbia cast albums (like Company, etc.) He has a biography coming out this year which I look forward to reading, but I did talk to him some years back on Facebook about the remastering. He said in some cases he was thankful to be able to go back and "fix" things--notably for Company: It opens with a phone busy signal, and then the orchestra comes in echoeing that same busy signal on their instrumentsl. It always drove him crazy though that the orchestra and the actual phone busy signal were in different keys--so for the remaster he fixed the phone signal so it would be in the same key as the orchestra (which now sounds normal to me, but was a big change when I was so used to the previous CD release.) For Gypsy he actually used several alternate takes that he thought sounded more spontaneous or theatrical (he talks about this in the liner notes for that release) and he DID say to me on DV that he sorta regrets doing that in hindsight, just because it now strikes him as hubris to second guess what his mentor, Godard Lieberson (who basically created the modern cast album) originally decided on.