From what I have read, the Mags are great pumps except for the 36. I have 2 Mag5's and a Mag7.
I did replace the Mag5 impellar with a Mag7 and it did increase flow. The impeller is not perfect though. With the housing screwed back on, it rattles a bit. Not sure why. I only put one screw back in it and it is running my frag
tank with no noise. It is submersed of course.
Can be done though. I am not sure about the bigger Mags though. Try it and find out. The impellars are usually around $10
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,