I haven't heard or seen the new Roundabout production yet. I hope to see it in the next few weeks.
So I can't really comment on Jason Carr’s new orchestration. I'm sure the choice of 5 pieces instead of the original 11 was dictated by economic considerations (as my 11 piece band in the 80's was also sized down from the standard Broadway orchestra of 25 because of budgetary considerations.)
I have heard that the original production in London was prepared for a pub theatre - the equivalent of our Off-Off-Broadway. I'm sure his 5 piece band was just the right size for that venue.
(In the same vein, Assassins was done at Playwright's Horizons in the 1990's with just 3 pieces for an Off-Broadway tryout. But when produced by the Roundabout in 2004 it was orchestrated for a 13 piece band. One has to wonder why the Roundabout didn't see the need to expand the band for the change in venue this time.)
If there's a guilty party here, it's the person (producer?) who felt that Sondheim's magnificent score was not worth the money it would cost to hire 11 musicians to play for audiences paying top dollar for tickets.
A new production should create a fresh approach to the physical production, sometimes including new orchestrations. But usually the new orchestrations for a revival are nothing more than an attempt to save running costs by using less musicians. This discussion should have been about Jason Carr's different choices with the 11 piece orchestra he should have been given.
And I'm sure when I hear his work, I will be appreciating how well he did with limited means - a feat I've often tried to pull off myself.