A check of the tastybrew calc shows Munich to be around 3 percent lower in the gravity. I don't think it should impact your effeciency by 10 percent.
For me Munich is an expensive speciality grain and I have not gone over using 2 lbs in anything.
% (lbs) Grain Gravity Points Color
50.0 % 5.00 American Two-row Pale 27.7 1.8
50.0 % 5.00 American Munich (Light) 24.8 10.0
I did not think that Vienna or Munich and other specialities had been modified so much that they needed standard grains. But that might explain why a Vienna and Wheat brew I like always has horrible efficiency, maybe I could save a few pennies by going with 3lb 2 row, and 2lb Vienna.
Or maybe you just had a fluke brew, but best of luck.
In Primary: Belgium Chimay clones.
In Secondary: Braggot, pale ale, end of the world white.
Conditioning: Mead, Cider, braggot, Belgium Wheat.
On Tap: Clones, Chimay Blue, Red, Porter, malted cider.
Bottles: Far, far, too many to list.