Which one would you prefer? I feel like however fast the FAC goes, in close proximity they will be very vulnerabke to other elements. Whereas with higher displacement and endurance, they can be more versatile and take different missions around mediterranean.
The ship may travel at high speeds with either diesels or gas turbine, latter being more compact in size. But it is about reaching to that speed regime, and gas turbines can be ramped up pretty much quickly whenever you need while diesel engines need to be ramped up by steps, and warming up. The warming up time is much shorter for gas turbines.
Besides its compactness this is another reason for adopting gas turbines. Allowing you to dynamically increase power in emergency cases, emergency maneuvers etc.
Seakeeping is important as well and both of those effect the survivability in similar ways. While one renders the ship inoperable at high sea state, another increases its vulnerability by cutting down the agility by removing gas turbines.
In my opinion the size could go larger but should definitely be equipped by gas turbine + diesels. The application in Water jet propelled ships are rather straightforward and does not involve cross-connecting gear, the installation is straightforward, so is maintenance. For people who may call gas turbines high maintenance, hard to up-keep; The application is not same as conventional CODAG or CODOD. There is not gears connecting Diesels to Gas turbines. Each individually positions as:
Diesel Engine - individual Reduction Gear - Steerable Water Jet
or
Gas Turbine - Individual Reduction Gear - GT Connect - Booster Water Jet