This thread never actually wound up explicitly spelling out working 300kbps settings so I'm sharing mine for others who are looking to break into this speed. I make no promises that these are 100% optimized but they get me 300+ feet which is all I need.
const uint8_t CONFIG[][2] = {
{ REG_OPMODE, RF_OPMODE_SEQUENCER_ON | RF_OPMODE_LISTEN_OFF | RF_OPMODE_STANDBY }, // 0x01
{ REG_DATAMODUL, RF_DATAMODUL_DATAMODE_PACKET | RF_DATAMODUL_MODULATIONTYPE_FSK | RF_DATAMODUL_MODULATIONSHAPING_00 }, // 0x02
{ REG_BITRATEMSB, RF_BITRATEMSB_300000 }, // 0x03
{ REG_BITRATELSB, RF_BITRATELSB_300000 }, // 0x04
{ REG_FDEVMSB, RF_FDEVMSB_300000 }, // 0x05
{ REG_FDEVLSB, RF_FDEVLSB_300000 }, // 0x06
{ REG_FRFMSB, RF_FRFMSB_915 }, // 0x07
{ REG_FRFMID, RF_FRFMID_915 }, // 0x08
{ REG_FRFLSB, RF_FRFLSB_915 }, // 0x09
{ REG_RXBW, RF_RXBW_DCCFREQ_111 | RF_RXBW_MANT_16 | RF_RXBW_EXP_0 }, // 0x19
{ REG_DIOMAPPING1, RF_DIOMAPPING1_DIO0_01 }, // 0x25
{ REG_DIOMAPPING2, RF_DIOMAPPING2_CLKOUT_OFF }, //0x26
{ REG_IRQFLAGS2, RF_IRQFLAGS2_FIFOOVERRUN }, // 0x28
{ REG_RSSITHRESH, 220 }, // 0x29
{ REG_PREAMBLELSB, RF_PREAMBLESIZE_LSB_VALUE }, // 0x2D
{ REG_SYNCCONFIG, RF_SYNC_ON | RF_SYNC_FIFOFILL_AUTO | RF_SYNC_SIZE_2 | RF_SYNC_TOL_0 }, // 0x2E
{ REG_SYNCVALUE1, 0x2D }, // 0x2F
{ REG_SYNCVALUE2, networkID }, // 0x30
{ REG_PACKETCONFIG1, RF_PACKET1_FORMAT_VARIABLE | RF_PACKET1_DCFREE_OFF | RF_PACKET1_CRC_OFF | RF_PACKET1_CRCAUTOCLEAR_OFF | RF_PACKET1_ADRSFILTERING_OFF }, // 0x37
{ REG_PAYLOADLENGTH, 66 }, // 0x38
{ REG_FIFOTHRESH, RF_FIFOTHRESH_TXSTART_FIFONOTEMPTY | RF_FIFOTHRESH_VALUE }, // 0x3C
{ REG_PACKETCONFIG2, RF_PACKET2_RXRESTARTDELAY_2BITS | RF_PACKET2_AUTORXRESTART_ON | RF_PACKET2_AES_OFF }, // 0x3D
{ REG_TESTDAGC, RF_DAGC_IMPROVED_LOWBETA0 }, // 0x6F
{ 255, 0 }
};