Stock pump uses 26 watts, and the maxi-jet 900 uses 14. The maxi-jet seems to pump much more water as well. If you don't need more flow, you may get the same performance of the stock pump with a MJ600, which will presumably use even less power.
I'm a bit peeved that the MJ uses so much, it claims on the label to use 8.5 watts, which is a figure I have seem bandied about the net quite a bit.
For the record, these test were done in a 10g bare tank i use to mix salt water. The water was cold (60 F) and was not saltwater. I have no idea if any of this affects the numbers.
But as to your problem, i would think that trimming 12 watts would proably save you quite a bit of heat. It might make a 2 degree difference or so, I'm not really sure. How much would a 12 watt heater that's alwasy on raise the tank temperature? Enough that you'd want to unplug it if you could, I'm sure.