I'm under the impression (no personal experience, yet) that canister filters have lower flow rates because of the filtration provided it doesn't need to be as high. The XP3 is rated for something like 175gall aquariums, at 350gph. I think this is probably plenty. Going for the 600+gph pumps, while isn't necessarily a bad thing, is probably a lot more money than you need to spend.
just my thoughts.
I'm debating right now between an eheim and a filstar for my 46gal.