Actually, according to the original data from Alu-Rex, the maker of Gutter Clean and the licensor of the product to Alcoa, it will handle 305" per hour.
Much like a clean screen, it actually has a siphoning effect which pulls the water down into the gutter.
"Reverse curve" products can rarely handle more than 8" per hour. Of course, the average rainfall in the U.S. is only 30-40" per year . . . and only Louisiana gets more than 6" of rain . . . per month.
Leaf Relief is an excellent product when it is clean and free of debris. Alcoa is a superb company with great marketing and distribution, and they clearly have VERY loyal employees.