While I agree his overall production might settle things down, I disagree with a lot of your assessment. His production last year was very solid. 1000+ yards, great AVG, and 13 TDs. Prior to that, he was sitting behind Boykin, and if you know Kelly at all, you know he isn't big on playing young skill players.
That said, he still had 600+ his JR year with Boykin as the headliner. As far as big games, IMO he did just fine. Had a good game vs UGA, and was a monster in the bowl game this year. The only other "big" game was scUM, and everybody looked bad. Book was 8 of 25... Worst game plan I've ever seen from Kelly.
Then add in the other impacts. After they booted Long and Rees took over, his production jumped up significantly. Also just the fact he has Book throwing it to him. I love Book, but he's a three star guy with clear limitations. It'll be interesting to see how he does with Tommy as the OC in 2020.
As far as separation is concerned, the great majority of his routes were possession routes and 50/50 type stuff (so didn't get much "designed" separation). Book didn't throw a lot of go route stuff either. His hands were more than good intermediate and deep. Drops (true drops) for the most part were short stuff that can arguably be a "touch" and timing issue to an extent with Book. When they started running him a lot in shallow routes (when they moved Finke outside for a time, which was dumb), he was not near as productive. I'd say he can easily overcome those issues by improving hand readiness on the short stuff. That can be taught.