Hoi zäme,
if anyone was wondering what the gain on the raspi 3B+ might be in terms of WiFi strength, I can tell you that for my setup without changing anything else than going from Raspi 3B to 3B+ the RSSI changed from -72 to -54. I was wondering if that isn’t dBm (as it says in the weppapp btw), but the name of the param sent via MQTT is named RSSI.
Off topic but interesting: My Raspi worked since almost 4 years now and saw some quite high temps during summer (e.g. 76 degC soc Temp).
Cheerio!