As I understand the situation, Sparkfun promotes the LowPowerLabs library, while we use JeeLib. The message format of JeeLib is incompatible with the inbuilt packet mode of the RFM69 (which is why we and JeeLib need to use it in the RFM12B ‘compatibility’ mode) whereas I understand LowPowerLabs does use the native RM69 packet mode.
The essential difference between the two modes is that compatibility mode requires the processor to handle each byte/character separately, whereas in packet mode, the entire message is handled by the RFM69 and passed to and from the processor in chunks of up to 66 bytes.