I have a friend who has a big sump filled with submerged media, filtering perhaps 300 gallons. It has worked well for him for years. It may work perfectly for you, as well, so I wouldn't necessarily redo a working system. Just keep it in mind for next time.
As for flow rate through the filter, I again wouldn't worry about it just yet - if you want more water movement or surface agitation, you could just add a powerhead or two. Of course, this depends on what you keep...