I'll let you in on a little secret fijiwigi. The pumps in my sump stay warm enough that a heater is not even warranted. By the way, I replaced my Mag 7 with an old Catalina 1800, which was what my skimmer came with originally. Since then, my temp has been maintaining between 81-83 degrees 24 hours so far. I use a big temp guage so I can see if from a distance.
You're right. The sump is such a confined area that things can heat up rather quickly. If I were more handy, I would plumb the pumps to run external, but my luck they would leak and it would be a disaster and my wife would kill me.
Some people have the knack to do it, while others' have to pay to have it done.