Minimum reccomended tank size is based soley on water quality issues. Back when BH was the "best" food for them, 10 gallons per fish is what was reccomended because of how harsh BH is on the water quality. Now a days, there are better options available that are much more forgiving on the water quality. You could easily do 6 discus in that tank..if you stick with a commercial pelleted type of feed (and limit your frozen foods given). Water changes are still necessary and essential for a healthy fish. Oversizing your filtration definetly helps. A wet/dry filter is currently the best option for discus tanks IMO as it is very efficient and relatively cheap compared to some canister filters on the market. So long as your water changes are kept up, the water is warm, and you feed clean foods you can easily do 6 discus in that tank.
-Ryan