Why U-505 did not shape American submarine design

How would US submarine development change if U-505 had not been captured?

It would scarcely change; the boat's intelligence contents were valuable, but its design offered little to American fleet-submarine development.

By the time of capture, the Type IXC was neither the newest German submarine nor well matched to American fleet-submarine doctrine. The US Navy already had its own long-range submarine requirements and did not need U-505 as a model. Its major value lay in the intact Enigma equipment and other intelligence recovered aboard. Losing the capture would affect intelligence exploitation rather than the trajectory of US submarine hull design.