What Germany might have done with an I-400-type submarine

Did German admirals know about Japan's I-400 submarines, and how might they have used them?

Probably not before the war; Germany could have used an aircraft-free version as a very long-range torpedo carrier off America.

German officers probably knew little about the Japanese I-400 class before the wartime alliance matured. A more useful counterfactual removes the seaplanes and uses the large hull for fuel and torpedoes. Such a boat could have sustained a long and dangerous commerce-raiding deployment off the American coast. The answer makes range and payload, rather than novelty alone, the features that would have mattered to a German submarine campaign.