I do see the problem there Steve. You need 4 GTs for it, and fuel isn’t exactly cheap nowadays. Even the price of HFO has raised from 260$/t to over 300$/t nowadays and that is just HFO, add to it that the oil will only last for 27 years (worst estimates) to a maximum of 50 years (about the best estimate), which means you already have to take in account new methods nowadays. You can see this forward looking idea in the merchant navy itself, tankers are getting smaller as these huge tankers will not be needed in 30 years and as their servicelives are mostly aimed at an age of 20 to 30 (or even more if possible without too much sacrifices), it would be useless to still build 400+m tankers.
Navies will, in the near future, undoubtedly have to look around for fuel and economy (which is actually already happening). Carriers like this design might be propelled by two GTs and still make a sufficient speed.
Last ship we had a steam turbine of 36,000hp, to propell our ship to over 20kts without a problem(officially something like 22kts, but as the need wasn’t there, we didn’t do that, we did make 24kts over ground though). Ship was 277m longx43m beam, with a laden summer displacement of 105,000t. This turbine did guzzle around 120t of fuel each day (depending on what we were using).