Your list is solid, but a bit more analysis improves prospects.
Defending Iowa is not that tough. It’s Grassley. Lots and lots of years to build support since first election in 1981. He won 64.5% in 2010. However, he will be 83 in 2016.
Missouri tougher, but Blunt was in the House about 15 yrs and is a seasoned politician. He won in 2010 with 55%, replacing Kit Bond.
New Hampshire is Kelly Ayotte. Won in 2010 with 60%. That doesn’t look dangerous.
Ohio will be Rob Portman, who won in 2010 with 57% of the vote against the Dem Lt. Governor. No slouch. This doesn’t look dangerous, either.
Pennsylvania is indeed dangerous. Pat Toomey won in 2010 with only 51%.
Kirk in Illinois won with only 48% in 2010. Definitely a dangerous situation.
Johnson won in Wisconsin in 2010 with only 52% of the vote, but he did so over Russ Feingold, who should have been unbeatable. Wisconsin is changing a bit. This looks less dangerous for sure than IL and PA.
So yes, the GOP defends a lot of seats in 2016, but only 7 are reachable by the Dems. They will have to win 5 of the 7 and not lose either Nevada or Colorado.
The GOP, excluding a Hillary wave of some kind, is not really threatened in the 2016 Senate election.
Correction, only 4 of the 7 if they hold the presidency.
Good analysis, thank you.