Sump advocate here, and I cant claim to know much about high flow cannisters, but can you name one that has even 600 gph at say 5 feet of head? And that gph still isn't anywhere near enough for a 120.
Just thought I'd chime in as an engineer.
Head pressure is determined by the net vertical distance you're moving a fluid. This is what drives a siphon - sure, the water has to go up a little bit at first, but the net change in elevation pulls it down and out.
For closed systems, such as a canister filter, head is the vertical distance from inlet to output. With a typical canister filter, the net vertical distance you're moving the water is close to nothing, so you'd have a couple inches of head.
As some others have mentioned, the presence of media can impede flow. Additionally, the tubing can cause some frictional losses. If you had to lengthen the filter's lines, you might reduce flow that way, but head pressure is not a significant part of the equation.
Edit: Also, the Eheim Classic 1500XL
is rated for 2400 L/hr, which equates to 634 gal/hr. I'm not sure how much the presence of media will reduce flow, but it's surely still in the ballpark of 600 gal/hr. Also, that rating is at 50hz. I don't quite remember this from my one electrical class, but I believe 60hz power as found in the US would increase speed a bit (don't quote me on this).
Edit 2: Some quick googling says that 60hz power will drive a 50hz motor 20% faster
. So, unless there are other effects at play, the 1500XL can do 761 gal/hr without media in place. Assuming a 20% reduction in speed with media still gives you a hair over 600 gal/hr.