My MLT is 20 quarts, which is a little too snug for a 10 lb. grain bill. I do it on a regular basis, but it requires a thick mash and careful stirring. If you ever find that your mash capacity is too low (for a barleywine or whatever), just add some DME to bring up your OG.
Did you find a cooler with smooth sides, or does it have grooves in it to hold dividers? I keep looking for a bigger cooler for a MLT, but they all seem to have grooves in the sides.