True that Iowa was very efficient offensively, but I think the argument that they can maintain a high offensive efficiency while being decent on defense is a stretch.
1. Getting to mediocre is a massive leap. Iowa was absolutely atrocious defensively. It's going to take a HUGE improvement to even be mediocre.
2. When you play oddball lineups like Fran tends to do you are creating mismatches on both ends of the floor. The more unconventional a lineup is in terms of positions, the higher the offensive efficiency and lower the defensive efficiency. If he's going to play more conventional lineups to match up and guard better, offensive efficiency is going to take a hit.
3. Similar to #2, I know people think it's a matter of simply playing harder on both ends of the court all the time, but there is simply a tradeoff. Players can either coast a bit on one end of the court, or you can get fewer minutes from your best players.
4. Yes there are some players on the roster that physically could be good defenders, but against major conference competition Iowa's current roster is usually going to be the less athletic team on the court as a whole.
5. There's nothing in Fran's history that suggests he's going to have a good defensive team, or that a majority of the players will defend at a level their length and athleticism suggests they are capable of.
I'm not saying it can't happen, but it's a whole hell of a lot more than a few guys playing harder and a couple incremental improvements in scheme and team defense.