You mean these ships.
http://en.wikipedia.org/wiki/USS_Narwhal_(SS-167)
http://en.wikipedia.org/wiki/USS_Nautilus_(SS-168)
I think they were re-engined and modernized. The article for Nautilus says
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In July 1941, she entered the Mare Island Naval Shipyard for modernization — radio equipment, external torpedo tubes,[9] re-engining (with Winton diesels),[11] and air conditioning
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The overall range of the ship not that far off, 9,380 nmi @ 10 kn vs 11,000nmi @ 10kn for the Gato class. Yes top surface speed was lower but with the way the submarine campaign was waged I don't think that was that significant.
Both Narwhal and Gato had a 300ft test depth so I don't think they were deficient in that area.