Keep in mind with a big beer like that (21 lbs for 5 gallons) that your efficiency is going to drop unless you use more sparge water. The problem with using more sparge water is that you have to boil longer to get the volume you need, and there comes a point where you're wasting dollars of fuel to save dozens of cents of grain. For example, Kal's Electric Brewery system gets 95% efficiency in general, but on a big barleywine, it dropped to 86% efficiency
The reason is that you have less sparge water available (assuming you want a reasonable pre-target volume that you don't have to boil forever) due to requiring more strike water to maintain a reasonable liquor-to-grist ratio, so it's less effective in rinsing out the grain.
I'd suggest brewing a beer with a 1.040-1.060 OG rather than judging by a such a big beer.