In my 48gal i have about 800GPh(17X turnover) but it's LPS and every thing is happy i'd need more if i wanted SPS..
I think you want to shoot for aleast 30X turnover for SPS..
2 MJ1200s would not be enough for sps. The DIY mod is a little tricky. I find the Sureflow kit to be great, but you really need a magnet as well because the extra flow is pretty intense and suction cups can fail and result in a sand storm.
all you really have to do is drill some holes in the bottom of the housing n saw off the circular output so that it's wider... n ive heard some stories about the mod eating smaller fish