So, I got myself a wAP ac. To make its 5 GHz radio cover the whole flat, I did not limit its tx power output and configured it to use the 5745 MHz channel (which is rated for 30 dBm here in Russia). The 2.4 GHz radio is turned off (my pre-existing RB2011 has a a more powerful transmitter) and the CPU usage is pretty low. I'm using the stock power supply unit and PoE injector; voltage meter says 26.5 V. Ambient temperature is ~25 degrees Celsius, i. e. well within the documented limits.
After 30-40 minutes of calm uptime (one client, no heavy loads) I noticed that `/system health print` says 61 degree Celsius. After 5 minutes of iperf3 it heated up to 70 degrees (and continues towards 80 as I'm writing this). Is that OK?
Update: so, in ~800 seconds of running iperf3 it heated up to 85 degrees Celsius and apparently stabilized at that temperature.