Very useful new feature.
I learned a lesson: I'll be using the nrf24 without booster from now on. Much better range and commmunication.

output from nrf24 with radio set to LOW:

-256 :transportGetReceivingRSSI()
-29 :transportGetSendingRSSI()
-256 :transportGetReceivingSNR()
-256 :transportGetSendingSNR()
-12 :transportGetTxPowerLevel()
25 :transportGetTxPowerPercent()
0 :transportInternalToRSSI(_transportSM.uplinkQualityRSSI)

I added this to my sketch and observed serial output:

Serial.print(transportGetReceivingRSSI()); Serial.println(" :transportGetReceivingRSSI()");
Serial.print(transportGetSendingRSSI()); Serial.println(" :transportGetSendingRSSI()");
Serial.print(transportGetReceivingSNR()); Serial.println(" :transportGetReceivingSNR()");
Serial.print(transportGetSendingSNR()); Serial.println(" :transportGetSendingSNR()");
Serial.print(transportGetTxPowerLevel()); Serial.println(" :transportGetTxPowerLevel()");
Serial.print(transportGetTxPowerPercent()); Serial.println(" :transportGetTxPowerPercent()");
Serial.print(transportInternalToRSSI(_transportSM.uplinkQualityRSSI));Serial.println(" :transportInternalToRSSI(_transportSM.uplinkQualityRSSI)");

I don't know if it's a valid way to check the values.