According to this source:
The noise figure of the Si446x family of chips is typically 7.5dB in the 868M/915M band, and about 6.5 dB in the 315M/434M bands.
The noise factor is easily computed using the formula F = 10^(NF/10) where F is the noise factor and NF is the noise figure in dB. For NF = 7 dB, F ≈ 5.
I would suggest adding a first-stage pre-amplifier with a low noise figure right after the on-board antenna. According to Friis’ equation for noise, this will significantly reduce the total noise factor of your system, which would otherwise be ~7 dB with the transceiver’s seemingly-poor LNA. Even if your link budget appears to work out without it (e.g. due to a high-gain (but noisy) LNA built into the transceiver module, P_Rx can be greater than –126 dBm), that does not guarantee your system link margin (i.e. your “signal-to-noise ratio” in simple terms) will be very high, or that you will obtain a high data rate equivalently. This is because you are not taking noise temperatures into account, but only signal power.