I have a much smaller tank (135 gal) running an OM 4 way with two bottom and two rear returns. I had difficulty getting a decent wave action. Perhaps two four-ways or an eight way will work better.

What I ended up doing that worked very well was adding a Wavebox. How about 2 4-ways and a dual wavebox? If you cound aesthetically get the wave boxes incorporated, they create great action, and the CL will keep the suspended debris into the water column where it can be skimmed away...