I-400 was a abnormally large submarine.
IJN A type subs were large (larger than US fleet boats) but not as large as the I-400
But generally Fleet Boats were hotels compared to U-Boats.
They had air conditioning (which was turned off when submerged however, to conserve battery power), showers, refrigerated food, water distillers that could provide enough water for both batteries and crew, and sometimes even a ice cream machine!
Yeah, thought that was understood, but theses were specialized one-offs basically. IIRC we sank them to keep from giving some to the USSR as war booty.
More like it was felt nothing could be learned from them anyway.
The Soviets got dozens of brand new german boats and the Japanese were way behind in submarine technology.
The catapult/hangar solution was some inspiration for Regulus submarines, I think.
3 "I" class were completed, as Opposed to 77 "Gato", 128 "Balao" and 12 "Tambors", 10 "Sargos"
The "I" class is more in the same category as the US Nautilus and Argonaut, specialized subs
on a related note, now deceased but famed USA author Clay Blairwas a submariner during WWII. I don't know what sub he was on or if he saw combat. Clay Blair is respected for his biographies of many well known WWII people, including Gen. Omar Bradley, and Gen. MacArthur, as well as many books on WWII.