Sabrina, although it doesn't feel like it, you are probably getting better mileage then that Jetta would on the same trip.

A block heater would speed up the time needed to bring the engine up to full temperature, but the engine also runs to bring the catalytic converter up to temperature for emissions reasons. I'm not sure that you would hit that point on a 2.5 mile commute.

The TCH works best after everything is up to operating temperature.
I have about a 15 mile commute and take the occasional longer trip. The cold weather knocks 10-20% off my mpg depending on how cold it is. I've found that the TCH works best at 60-80F and on trips of more than a few miles that allow you to run at 35-40 miles per hour or 55-60 miles per hour.
For me, the more highway driving I do the better mileage I get. My best was a 63 mile mostly highway trip one evening at 70F where I got 47.7 mpg.
If the hills aren't too bad, on good weather days have you considered a bicycle for the 2.5 mile trip?