Efficiency is the other concern, especially if you batch sparge. Expect at least a 15-20% drop if you go from a 10 lb to 20 lb grist (batch sparging), all else being equal and collecting the same amount of wort.
Boy, I sure haven't seen anything like that. Maybe 5, sometimes 10 points.
From Kai's site: http://braukaiser.com/wiki/index.php/Batch_Sparging_Analysis#Effect_of_the_grist_size_.28weight.29
According to the mathematical model of batch sparging, the efficiency is dependent on the amount of grain used, pre-boil volume, absorbtion ratio, dead space, number of sparges, and relative runoff sizes, provided all other variables are well controlled and 100% conversion is achieved. If only the amount of grain is being changed, then the model says there will be an 17% difference between the two for a 6.5 gallon (temp adjusted) pre boil, at least if my calculation is correct. I have found this model to be a bit on the high side of estimated efficiency, I usually get a few points less than predicted.
Personally I've only done a couple of *really* big beers, so I'm not the person to look to for verification of theory. On the last one efficiency went from a normal 80% down to 65% IIRC, collecting 7 gallons preboil with 23 pounds of grain. Denny, I seem to remember you saying that you usually have a bigger preboil when you do the barleywine? Far be it from me to question a brewer that has brewed a *lot* more batches, it's just hard for me to wrap my head around only a 5% efficiency reduction when going from 10 to 20 lbs of grain, all else being equal