I run a 10000 to my skimmer (used to be 2 Eheim 1260s) at level 4, a 10000 as my return (used to be a Mag12) at level 4, a 5000 to my reactors, and a Barracuda as my closed loop.
I know they are quieter and use less power (as per my Apex) but I know one of the 10000s wouldn't keep up with the Barracuda, I'm not sure 2 would even keep up but I may test this soon
. They are nice pumps but may have a bit of a exaggerated GPH rating.