I just switched out two horrendous Powersweeps for two MJ 1200's (Yeah!) on my 20g. But, and maybe this is because I'm not used to so much flow, seeing those in action has me slightly hesitant as to whether I overdid it or not.
what are you keeping. That is about 30X trnover which will be good for SPS in a 20g. If you look at the MJ900 it is only using 8.5w compared to 20w on the 1200 and only 65 more GPH. I think you will be fine just make sure you are not blasting corals with the flow from the 1200. Random flow is better by bouncing it off the glass and corners and things. HTH