SM7B with Elgato Wave XLR MK.2: what do the 80 dB of gain actually buy?
Yes. At speaking level the SM7B needs about 59 dB, and the Wave XLR MK.2 publishes 80, so 21 dB of gain is spare. Those are the published figures.
The Shure SM7B has enough preamp on the Elgato Wave XLR MK.2 for a normal speaking voice. You do not have to add a box in front of it.
Shure rates the SM7B at -59.0 dBV/Pa, the voltage from a loud 94 dB tone at the grille. A speaking voice is closer to 85 dB, so the preamp still has to find about 59 dB to land near −9 dBV. That −9 dBV figure is just a tracking level: high enough to use, with space before the converter clips.
Set the knob for the voice you actually record and leave a little travel for plosives, instead of pinning it because the spec sheet has a higher number. A booster on a chain that already has about 21 dB spare only adds a box. Most of those boxes also use the phantom power themselves, which will silence a condenser.