Just guessing here, Ross.
The factory deep sump pan measures 7.5" deep and holds 6 quarts.
The Canton race pan measures 7.5" deep and also holds 6 quarts.
The Milodon race pan measures 8" deep and holds 7 quarts.
Figuring that most deep sump race pans are basically square and have the same general volume for the given area (unlike the stock pan which has an angle to it, changing the volume), I'd guess the pan holds roughly 9 quarts. All the above does not include filter capacity, so I'd guess you are spot on.
9" is pretty deep. I'd be leery of using it on anything other than a truck or gasser style car, but I'm sure you already know that.
I spent all morning measuring pan depths to order a new deep sump pan for my car, so it's kind of ironic that you posted this question today. Side note: Summit and Jegs did not have the Canton pan that I wanted, but Amazon did...and it was cheaper. Go figure. They also had the Canton oil accumulator, electric solenoid kit, mounting brackets and pressure cut-off switch that I needed, and all cheaper than Summit, and all Prime deals. Is Amazon going to be the new performance parts dealer?