Maximum achievable SNR (SNR before LNA)
I think that you do have to compare powers and also in a certain bandwith. The noise is a power in a certain bandwidth, if you choose a smaller BW the noise will be less !

If your signal is only 1 MHz wide, you don't need your the 100 MHz bandwidth (the LNA could be 100 MHz but after mixing you would filter to a 1 MHz bandwidth and get rid of the extra noise).

So you cannot compare your 50 mV signal to the noise power (in a specified BW). You need a signal power (in a specified BW) and compare that to the noise (in the same BW).

Think of it like: if your signal is only a 50 mV sinewave it will use a very small BW (it is only one frequency). Now compare that to for example a WiFi signal which uses many frequencies in a certain BW. Combined they can also peak at 50 mV but they would contain a lot more power (and information!).