Well, I think it's not completely true that he did "very little" - but I think it's almost certain that his Cass research was summaries. The Birds of Prey detail is a pretty deep cut, I think, so there was some level of knowledge going on.
I do like 21 and 28, even if 28 feels really odd with how Steph enters and exits without much of an impact on Cass's status quo. And I really wish I could like 27 - it's actually one of the first 20 or so comics I remember reading, back in college in 2004ish - but it really biased me against Steph, and when I came back to it much, much later, after falling in love with Steph through War Games, War Crimes, Dixon's second Robin run, and then her run as Batgirl, it really doesn't work for me on a purely Steph level. On a Cass level, it works fine, and the art is gorgeous, as it is Phil Noto. I feel like I'm one of two or three people I've met who actually dislikes it, though, so definitely in the minority.