Expectations from factory block heater?
 

What is realistic to expect in terms of coolant temps?

I plugged mine in yesterday for about 3 hours at 15°F outside, and there was not a really noticeable decrease in the warmup time (about five minutes driving).

Any blocker heater only warms the oil itself up enough to keep it a bit thinner than when really cold. None will have any noticeable effect on overall engine warming time.

If it's an actual "block" heater, aren't those installed by removing a welch plug from the engine block and inserting the heating element into the water jacket? It likely doesn't warm the oil as much as the slight thermal expansion loosens up the piston bores a little to allow for easier cranking. When we were having to use block heaters, we didn't care about how long it took to warm up, we were just thankful it started at all! I don't recall that the engine warmed up much faster. They were all cast iron with very little aluminum. Not quite the same as our boxers.

If it's the same as the one Subaru puts in the FB motors it should make quite a difference in warm up time. I had one installed in my Crosstrek at the dealer before I picked it up and that thing was fantastic. I'd plug it in at night when I got home from work and the car was warm and it would keep the water circulating all night. You could hear it and feel it moving through the upper rad hose. Even if it was below 0(F) I could get in the car in the morning and it was ready to go and the heat would come up immediately, the blue light on the dashboard wouldn't even come on.

The ones that circulate the coolant do a pretty good job. The heaters that are just an element help, but not enough to start your car and have heat.

The Subaru heater for the BRZ is just an element, A091SCA001. The description says "Warms engine coolant to promote easier starting in extreme cold conditions". I wouldn't expect much more than that from it.

Yes. I gave a horrible description by trying to keep it too short.

Yeah I thought all the block heater does is just make the car easier to start.

They're definitely worth it once the temperature dips below -15c.
On my old SX4 with the battery in the condition that it was, it would be the difference between the car starting normally in two cranks, versus the world's slowest crank and the battery dying out.

It's been below -20 the last few days in Ottawa and I definitely wish I had a block heater for days the car isn't I'm the garage.

Thank you all. It's just the one that goes into the freeze plug holes. It definitely made for easy starting at ridiculous cold.