I think that some % of the complex carbohydrates would be washed from the grain at the lower temps before the temperature triggered the enzymes that break down these sugars to simple sugars that yeast are able to eat.
I don't know what that % would be, but to some extent, you'd lower your efficiency and produce a lower bodied beer.
I don't know the exact chemisty, just the higher level processes, but I do know that the difference between mashing at 148 and mashing at 156 is a good 5-10% in gravity points due to the same enzyme issues as above, so depending on how long you left your grain soaking in water while you were heating up to your mash temps, I think you'd be surprised how much you'd affect your efficiency, gravity, fermentables, and unfermentable complex sugars left over in your final beer.
I wouldn't mess with it.