I would wait to see another, colder month first. The difference given is small enough that it isn't really clear that efficiency has really fallen, although you have reason to suspect it has. If on the other hand you see similar results for one or more of the three coldest months of the year (corrected for relative delta T compared to the balance point), then start digging deeper. Not that you can't start digging into it now, I just wouldn't call the contractor until you have some specific things to point to. Having this past and the next month both come in on the high side would suggest a real problem.
I can't speak for residential boilers, but one thing I've noticed about new AC units is that they often have electrical resistance heaters on them to keep the compressor warm when not in use and thereby prevent starts flooded with liquid refrigerant. On two stage units in particular that is an incredible waste of electricity for at least 6 months out of the year. (The crankcase heaters run about 40W each, 24/7 based on my measurments and some descriptions I've found online.) During cooling months the heater elements don't run as much because the compressor(s) run periodically and the heaters only run after X number of idle hours. The solution to the problem is simple: during non AC months I flip the AC breaker to the off position. When it is time to cool again I flip the breaker on 8-12 hours before I anticipate needing AC again. I leave it that way until we exit AC season. Ideally they should be instrumented to do this sort of thing on their own...but the controls aren't that smart at present and instead seem to default to heat pump protection settings.