Here's a working config for 19200kbps, effectively 2.9x slower than the default 55.5kbps, so it should yield longer range:
/* 0x03 */ { REG_BITRATEMSB, RF_BITRATEMSB_19200}, //default:4.8 KBPS
/* 0x04 */ { REG_BITRATELSB, RF_BITRATELSB_19200},
/* 0x05 */ { REG_FDEVMSB, RF_FDEVMSB_25000}, //default:5khz, (FDEV + BitRate/2 <= 500Khz)
/* 0x06 */ { REG_FDEVLSB, RF_FDEVLSB_25000},
/* 0x19 */ { REG_RXBW, RF_RXBW_DCCFREQ_010 | RF_RXBW_MANT_24 | RF_RXBW_EXP_3 },
/* 0x3d */ { REG_PACKETCONFIG2, RF_PACKET2_RXRESTARTDELAY_NONE | RF_PACKET2_AUTORXRESTART_ON | RF_PACKET2_AES_OFF }, //RXRESTARTDELAY must match transmitter PA ramp-down time (bitrate dependent)