Twice the flow lol? Umm xp4=450gph fx5=600 450×2 equals 900gph, nice math. Unfortunatly for you fluval measured their specific gph with all the hardware and no media at 606 and they also measured JUST THE PUMP ONLY WITH NO HARDWARE AT 900+. And the more media and sponges the less gph on top of that. So yours if full with media runs apprx 400-500gph. My xp4 loaded with media which i measured with stopwatch and 5gal bucket was right at 395gph.
After much research before buying, the xp4 has less flow restrictions in its chamber design allowing more media with minimal flow loss and less watts as well, the fx5 is great and user friendly plus has 5gal chamber with alot of volume but the extra $ just didnt do it for me, 100gph extra wasnt worth it since i already have 1200+gph on my 90.
There are many debates which is better but im simply clarifying that the fx5's 900+gph was pump only no hardware and rated 600gph with no media or tubing, the xp4 was also rated at 450gph with no media or tubing.
But they both rock and im not saying the xp4 is better cuz on a scale of 10 the fx5 is a 9 and the rena an 8 and the extra 90-110$ isnt worth it to me but if i had mad cash id plly have it!
And fyi this is how they measure all aquarium pumps. Call the manufacturer and they will tell you or just have a friend that works in the industry!