You do not want to use the 457's on the Conqueror anyway. Even with the 'higher' firechance on the 457's vs the 420's - 12 guns will inflict more damage (and fires) compared to the 457's 8 guns.
Hardly anyone used the 457's and WG decided to move the guns onto Thunderer. A slighty better ship would be nice tho - f.ex. give Thunderer the same heal as Conqueror.... but I guess that would be a bit OP :P
Another question: Is the Thunderer worth it? When Georgia first came out the game was full of these ships, so far I've only seen one Thunderer (and the guy that ran it made it look like a joke)
I got enough coal for this steelbeast, and want to buy her. However, she is 'just' a Conqueror with 457's, faster rudder, DEF AA, and worse heal?