I can see where the grill cover would help immensely in the winter, the ICE seems to cool VERY quickly when you get out in the cold air.
I'm down to 42-43 on my work commute (was 50+ in the summer). I thought it would drop even more as it gets colder, but now I'm not sure. I've found the heater doesn't matter much unless you're stopped. At least in my car, the engine runs pretty constant anyway for about the first 3-5 miles regardless of whether or not I have the heater turned on. Even when it goes into EV mode after that, the threshold is much lower to kick back over to ICE until the car is fully warmed, which seems to be close to 10 miles in (about 3/4 of my trip). Once the ICE is warm, the heater blows enough warm air for me to coincide with when it needs to recharge the battery. If I'm at a stop sign, the car will shutoff as long as it's after it's initial warmup stage and I don't have the heater on.
Yesterday I tried pulsing and gliding on the back road for the final few miles of my trip in the morning. The past few mornings I was showing 33-34 mpg on the instant for the drive in, and yesterday I got 38.5 even though it was the coldest day of the week so far. I'm going to try to keep doing that so long as theres no traffic.