I had a 55g with two Emp400's. I took the cartiges for one of them, threw them away and made my own, essentially making one a mechanical filter only. The other 400 got filled with ehiem effisubstrate pro. These filters hold ALOT of bio-media and coupled with the bio-wheel you would be surprised how clean the water was in my tank of 30+Africans. 800gph in nothing the scoff at! drsfosterandsmith has these 400's occasionally for under $50. I got one on craigslist new in box for $40. Even if you pay full retail, the 400 vs. the 280 price will be negligible when considering the flow cap of the 400.
Good luck with some great HOBs!!