If it is the same maxijet mod I've been watching for the last almost year or so, you can get around 1200gph out of a MJ1200. 2-3 should be sufficent for a 120.
However you are right that the MJ900 is more efficent in terms of flow per watt.
Sue if you have pictures of the shroud can you post? I tried to make one, but it came out too ugly for me to risk putting in my tank
