SiameseMoose, thanks for sharing your experience. While I've never actually done a one-after-the-other experiment with the same recipe for two different mash times as you did, I did make 6 small batches of different beers with differing yeasts mashing in a range from 20 to 35 minutes, and in half (3) of those cases, the attenuation suffered significantly. Surprisingly, the attenuation also did not seem to suffer in the other half (the other 3) of cases. I chalk this up to variability in runoff and sparge times, not to mention ingredients, which for me were not constants. But once I started mashing for 40 minutes, all attenuability problems magically disappeared. I have not had one single problem since. I imagine this minimum mash time number is different for different systems, so brewers beware -- your mileage may (and probably will) vary at least a little bit.
Thank you for not typing "YMMV", man that annoys me
I usually mash for 60 minutes and I always seem to get crazy attenuation no matter what my mash temp was. I've had 80+% attenuation with a mash temp of 156F, of course that could be because I used US-05, but still. Maybe I'll try mashing for 45 minutes on my next batch for kicks.
There's some great information in this thread.