PA means power amplification. Power amplification works a bit like the volume button on a phone meeting with bad sound. Turning up the volume might make it easier to hear the other participants, but it will also amplify any noise and disturbances.
In addition to that, power amplification stresses the power supply a lot (at least when sending), which often leads to failures.
So power amplification might actually make things worse. But it can also make things better, given that the power supply is good enough, the radio is shielded from noise, etc.
Data rate (250kBps) is set by MY_RF24_DATARATE. Default value is RF24_250KBPS, see https://github.com/mysensors/MySensors/blob/76dea1532ce2bcf21a5a167e21b4e0ba4eeaa51a/MyConfig.h#L550