mm receiving an incoming hit for 50 points of damage, the improved shield feat at a 5% increase, would indeed be worth 2.5 extra points of DR.
I was thinking about a build like this for one of my casters but I'm not entirely convinced of the tank usefullness as of yet. I think you are trying to force a class into a role it was never meant to fulfill. That's why I was thinking the feat was a bit over-the-top.
If you really plan on being main tank 90% of the time I understand taking the improved shield feat and hope you have a lot of fun doing so!