So, first: there is a principial differense btw dB & dBm. dB is a relative admeasurement. dBm is an absolut value of energy(power) which is measuring also in, e.g., W (watt). U shouldnt mix these values.
On my opinion & practice (in other radio PDAs but similar) there is no big difference btw levels -57&-65 dBm. There are a lot of another factors strongly affected on the radiolink, such us, e.g., sensitiveness of radioreceiver of AP in these channel, or quantity of clients & many others.