variables. Of course, shot blocking and rebounding correlate to defensive effectiveness for all teams but it is hardly sufficient and probably not dominant. Kaufusi was helpless last year on defense and BYU's D-PPP declined as Kaufusi increased minutes to a season low at the very end at 160th.
Austin in any year at BYU was more effective on D than Kaufusi was last year. Bigs do matter most for effective D, and this year we can expect that Kaufusi will not be outsmarted on so many possessions as he was in the final half last year. Corbin may play more minutes than Austin for reasons of durability but i expect Kaufusi will need another year to match Nate's defensive effectiveness.
Shot blocking matters, but you may be placing far too much weight on that one by itself. The raw shot blocker Kaufusi last year is exhibit #1.