I normally don't count JT Barrett in this sort of conversation because he's not really a first-year starter in the same way as 2018 Haskins, 2019 Fields, and 2021 Stroud. Barrett was recruited by the previous OC for his ability to perform in a very different offensive system, had already had 30-whatever games of starting experience, and already had three years of development (or non-development, thanks Beck/Warinner) under his belt. Barrett improved noticeably as a passer in his last year and that's a credit to Day, but I also don't really think he's a good analogy for what to expect from McCord/Brown either.
Personally, there's one very simple stat I keep coming back to: Three QB's have been first-year starters under Day (Haskins, Fields, Stroud). All three of them performed well enough to be Heisman finalists in that first year and were firmly established as "this guy is a future first round draft pick".
And that's basically what I'm expecting from McCord/Brown. Maybe there's some growing pains early in the season with McCord/Brown. Maybe the Bucks even lose a game because he's not ready to pull out a last second drive or makes a key mistake or etc. Maybe the run of Heisman finalists ends due to quirks in voting or whatever. But I do expect by mid-season, the QB will be a star and at least at the level where you can reasonably say "yeah, he should get Heisman consideration" and "this is a future pro".