Update 2: When I went to the store, they only had Domino's 6X in the 50lb bag. The manager offered to let me try out the 6X sugar while he tried to locate a 10X bag for me. He said I could bring it back if it didn't work. The 6X is way lumpier than any 10X PS I've used, so as Stitches described, it may have less starch than 10X.
I sifted it, and made a batch of MFF which turned out great!
Funny enough, he had called me while I was working to let me know he found some Domino 10X bags in the store.
I decided to stick with the 6X since it worked out fine (and I was in no mood to heave 45.9 lbs of sugar back to the store). I ended up using 3oz less than I used on a full batch of MFF I made with 10X. We'll see with subsequent uses if that was a fluke or if indeed I would need less 6X sugar compared to 10X.
So @tdovewings, Domino's 10X PS does come in 50lb bags.