I haven't done much detailed testing of my block heater, but I wouldn't be surprised if mine tops out at the same temperature differential. Still pretty sticky jazz to have the engine 65 degrees over ambient when you start it -- cuts out a lot of dirty, inefficient running on the way to operating temp.
Have you considered a lower grille block yet? I did mine a month ago, and still haven't seen temps over 210°F so it seems to be a worthwhile mod here in Iowa. And I'd be more than happy to turn on the heater if it threatened to go higher.
30+ MPG may be out of reach for our cars during the winter, but some smart mods and driving ought to keep it from getting too bad.