I agree it is good information too, except for one line "Using too big a transformer only reduces it's efficiency, ". That is not a given, there are too many factors when determining a transformers efficiency so that may or may not be true. Because the OEM transformers are usually "minimally sized" for the job, their load losses are pretty high. Load losses are exponential so going up in capacity by 25% can reduce load losses by as much as half, but usually at an increase of 25% in no load losses. No load losses usually account for about 20% of the total losses.
Total efficiency has to take in the no load losses which are 24/7 losses and the load losses, which although are higher are also dependent on duty cycle. If the transformer is under load 24/7, then the load losses are more important than a transformer that is under load for say only a few hours a day. In this case, I'd guess that the transformer is under load whenever the alarm system is on so it probably is closer to 24/7.
Materials, design and construction also play a big part in the efficiency. Better core material decreases no load losses and winding material (copper vs aluminum) wire size and design affect the load losses.
An indication of the transformers efficiency is the amount of heat that it gives off. That is total heat, not just the surface temperature of the transformer. All the electrical losses are in the form of heat.