After acquiring one of these, messing around with setups, and playing it for several battles in both GB and World at War, I have noted the following advantages over the Alsace:

-Slightly better spread. While it won't mean hitting with all shells often, or even an extra shell per salvo on average, the spread buff does help with high-offset shots, and other shots will hit with an extra shell or two enough to be noticed.

-Speed boost. When capped, the Thermidor can easily go 26/44 with a full 120 crew and not skimping on ammo, as opposed to 25/42.

-More useable displacement. It's slightly easier to keep the Thermidor at 26/44 than it is to keep the Alsace at 25/42, allowing more bulge. You can also tank on bulge to 200+ easy if you don't mind dropping to "merely" Alsace speed.

-Extra support slot (duh!)

The ship has the following disadvantages compared to the Alsace.

-It gains minimal benefit from removing the rear turret. Doing so only brings it up to 27/45. Even reducing ammo and cutting bulge, I had to remove multiple sailors to get to 28/47. An Alsace itself can get 27/45 with less skimping if the rear turret is removed.

-It has a noticeably larger hit box.

-It's missing the 2 large T slots amidships, with only 8 small and 2 large slots, as opposed to 8 small and 4 large for the Alsace.

The following gun options are available:

-Level 115 18.89"/45 cal Model 1942. These are the intended set, and have slightly better spread than the normal BB5 guns, with an ever-so-slight increase (0.04 seconds, or 0.2%) in reload time. The spread buff is more noticeable than the reload rate. Note that the ammunition for these guns has reduced space requirements - only 2 space compared to 9+ for all other 18.89" guns. As such, the ship can fit 5 binds. They retain the anemic range MN is so well known for.

-Level 101 18.89"/45 cal Model 1941. The intended guns for the Jean Bart, these weigh the same amount and take up 36 less space. However, the ammunition has normal space requirements, so you would not get any extra binds. These are pretty much identical to the regular Alsace guns in everything but space requirements. There is no real benefit to them besides the extra bind, and the only reason I'd use them was my gunners weren't high enough for the 115 guns.

-Level 103 18.89"/45 cal Model 1936. The regular BB5 guns for the Alsace, there is no excuse to use these whatsoever. They have no performance benefits over the 101 guns, weigh 15% more, and have a high space requirement that would leave the Thermidor with just 2 binds.

Performance in battle:

-Great Battle: It's just like the Alsace, but with more speed that allows it to be a little more aggressive a lot more often. When you rush in like an O-Project II but hit like a Monty, enemies die. Fast. The high damage and better spread also make the Thermidor better at defensive play than the Alsace as well, being able to hit slightly harder on average while also being capable of outrunning most BBs. The anemic range is still an obstacle, but the speed and hitting power still make this effective both offensively and defensively.

-World at War: It goes about as fast as the Charlemaign, which go 50 with my crew. At that speed, you can evade many shots with ease, and also run down almost any other BB, including most BB6s. In fact, you can even rush straight ahead at the start and get in range before the enemy scout even sees you, getting off a few quick opening salvos as you retreat back to safety. The guns still reload slowly, but a sufficiently overvetted crew can negate that to a large degree. Still, it can never have KM or RM level autocannons. All told, this ship is truly a beast in World at War.

-Line battles. The anemic range hurts this in those, and I would not take it into such a fight without a clear role. Its firepower can help defensive play, and its speed can allow for rushing, but it really needs to fit into an overarching strategy in order to to be worthwhile here.

Overall

The Thermidor is a fun ship that's basically an Alsace on steroids. It plays the same, but is strictly better. And while it's hindered by the lack of actual different gun options, it remains an excellent ship that can be played both aggressively and defensively, and shine either way. Still, I'd really like to see it with a new and completely different gun set it could use like many of the others..