I'm not sure why people are recommending the Bourg, after the cruiser plating changes the ship is simply not worth playing.
A pre nerf alsace with 12 380mm guns, even with the good shell ballistics and pen of the french bbs, simply cannot perform at tier 10 anymore. The only thing you do is punish broadsides and spam HE at angled targets. Wait a minute, that sounds like what the Thunderer does! Because the thunderer does everything the bourg does, except it does it better. Sure the reload booster on the bourg can help you kill something if you rush it, but that is dangerous at the best of times, and downright suicidal for your ship at worst. I have the bourg, and before the plating changes it was very fun, and I suppose you can argue it is still very fun. But with the, honestly, awful HP pool for a tier 10 bb (like 75K or less, cause I don't remember off the top of my head) and with thunderers being is the game, it is very hard to justify getting the bourg over anything else.
The shikishima however, is kinda of like the thunderer. Low barrel count but reasonably good accuracy that allows you to hit things. And while you don't overcome any important threshold by going to 510mm shells, your damage output is incredible. I would never recommend using a secondary build, simply because modern day world of warships is a HE spam fest, meaning that you're unlikely to do much with those secondaries by the time you'd want to use them. Yes the gun sounds are fantastic, but overall you want to get a ship that will perform will in world of warships today, and I do not see how you can pick a 380mm gun equipped bb over one equipped with 510s.