Iowa got bigger inside now having one legit sized Center with Olaseni standing 6-10. Also, isn't Aaron White almost 6-9? Yes, they are both freshman, but the give Iowa size off the bench that they haven't had for some time. Even Oglesby has some decent height coming off of the bench.
I don't believe the success or failure rests on Marble and McCabe, it really lies into how much Basabe keeps himself on the floor and that the bench sustain and or extend leads while on the floor. The other thing that will determine how Iowa does is Gatens, if he can start hitting the wide open shots he had last year, Iowa will be much, much better.
The true success of Iowa lies in how well they defend this year, they were not a good overall defensive team, but the 3-ball just killed them at times. If they tighten that up, they win more games.
You cannot pin the hopes of this team on two players when every player has a lot of improvement to make. Why not just say if Eric May doesn't lead the BIG in steals Iowa will be the worst defensive team in the league... If Marble and McCabe do get better then yeah, it will help Iowa tremendously, but McCabe won't be a starter, so it is confusing to pin hopes on a role player, same with Marble as he may not start.
The growth of each individual player will determine the success or failure of this team.