That's an awesome thought :) but as you say, now we need to also get a few hundred GW of generating capacity out of the Hoover Dam and then we'd have something. Hoover Dam is about 2GW nameplate capacity IIRC, so if we could now somehow dig out 199 equivalently sized new turbine halls underground around the dam we would have a real tourist attraction. That might look something like 398 Manapuori power stations. We would also need to install however much solar is required to both provide enough renewable power during the day to offset fossil sources, and have excess to pump enough water to store energy in our Extreme Hoover Dam project to power the country during the evening post-solar peak hours and through the night.

Is there a good study that explains how pumped hydro and solar can actually work to make a significant dent in our gas/coal power?

Some more spitballing:

Demand ranges between 400-650GW over a summer day (over 700GW in heat wave). If we look at EIA data for a summer week we see Hydro produces about ~50 GWh at peak, ~21GW at a low point, over a day. And we see fossil sources producing about ~270GW at minimum to ~460GW maximum over a day. Solar producing nothing at night up to 22GW then unfortunately falling away too early to contribute during the peak demand period (see the duck curve).

So the argument for pumped storage here seems to be that we can somehow get that 21GW to 50GWh production up to some meaningful number. Lets assume we can convert every dam in the country into pumped storage (obviously not but let's assume). Now as discussed need to increase the production capacity of hydro a lot. Let's say we can quadruple the generating capacity of every hydro dam in the country and turn them all into pumped storage. 200GW would be meaningful (not a full solution but nearly half way to a solution).

How? Sounds incredibly unlikely to me. Especially given not all dams are well suited to pumped storage anyway. You build new tunnels and pumping systems to get the water from downstream lakes back up. You add three more generating halls for every one, probably buried alongside the dam, how much is that going to cost? A lot. How long is it going to take? A lot longer. We need something that we can production line produce at this point.

Now if we wave a wand and somehow do that though, we could produce a maximum 200GW with our hypothetical hydro/storage set up. But we now also need to build however much solar is also necessary to reach our green 200GW target and pump that water back up during the peak solar period so the hydro can run through the non sunny part of the day giving us some 200GW of continuous Solar+Pumped Hydro base generation. That would have to be somewhere in the vicinity of what? I'm spitballing but maybe like 400GW of solar we need to install? How much do we add on for that cost? So we've quadrupled our dam's generating capacity at some incredible expense and built on top about 13x the amount of solar we currently have installed.

And we still have to keep fossil around to generate 70GW at night and 260GW during the day.

I just don't see how we get pumped hydro beyond anything more than a curiosity at this point. (That doesn't mean I think it shouldn't be pursued where it's feasible and the business case stacks up.)