These days a lot of people run quite mild flow through their sumps, and supplement that with lots of in tank flow via prop pumps. That said, I agree a mag5 sounds awfully mild for a 120. If you are going to replace it, I'd suggest looking into anything but a mag pump, they are power hogs, run hot, can be prone to having issues restarting, and overall just aren't all that great. They are popular because they are affordable, easy to set up, and everyone carries them.
Personally I'm experimenting with the newish DC pumps coming out of China, but they definately haven't stood the test of time yet, and the first generation ones are definately having issues so I can't exactly reccomend then (yet??).