I have the 15w version of the Hydor Slim in a 2.5g and it keeps the temp within the same range. So far, it does the job.
I tried out the 7.5w one and the 8w Marina Mini Heater and both didn't do much for warming up the 2.5g. Then again, it's also pretty cold here still and I am the kind of guy who does not like to put on the heat or have it too warm (I usually walk around my place in shorts and a tank top) so the general temp inside can get pretty low.
I do plan on keeping both the 7.5w and 8w heaters though, because they do a good job with heating up a small bowl if I ever need a hospital/quarantine tank.
Later on in the spring/summer I definitely want to look into getting an adjustable heater, especially since some pretty highly-rated ones are very affordable on Amazon and whatnot.