I agree about the sponge filters. I use them on my fry tanks and have one rated for 80 gallons on a 30 breeder(I believe your tank is 4 inches taller but could be wrong). However, I back that up with an Eheim 2211 which is rated at 40 gallons and 30% water changes 3x weekly. They are a good way of building up your BB and are very beneficial when tiny newborn fry are concerned. There is essentially no such thing as GPH with sponge filters. They are more about surface area. I would never filter a "display" tank with strictly a sponge filter but would highly recommend one as a second or third filter on a larger tank, just for that added BB. Comes in handy if one of your filters goes out or something of that nature. My advice to you would be to run down to the LFS and check if they have a larger sponge you could swap out on one of your sponge filters and just have 1 large one in the corner. 2 small ones just seem like they would take up space.
Finally reached 6 tanks... oh god, this addiction is getting serious. 75g mbuna tank, 65g gold severum, 55g OB peacock breeding colony, 30g(36x18) growout, 20 long Dwarf Alto's, 10g fry tank.
If I die young bury me in... An 8x2x2 with versa tops and a large sump for adequate filtration;)