There is a specific reason the US Navy hired the Gotland SSK for a while, and that was a concern at a weakness in their ASW ability when it came to detecting and destroying SSKs. There are numerous 'claims' of SSKs killing high value US targets in wargames, but it's often hard to sort the fact from the fiction in those respects. However consider this, during the Falklands war, one of the biggest time consumers of the Royal Navy was searching for two Argentinian submarines, one was a Type 209 and the other an old Balao class, we managed to attack and disable one (The Santa Fe, the Balao class) while it was at pier, but the other eluded us and caused the RN no end of headaches trying to find it, to the extend that HMS Brilliant torpedoed three whales after mistaking them for a submarine.
Furthermore, it was a warranted concern, the San Luis (the Type 209) had the opportunity to attack three RN warships during its patrol but fortunately for us, the Argentinian torpedoes were faulty and none hit.
So basically, just the knowledge of the existence of the San Luis in the region of the Task Force caused severe concerns and problems for the Royal Navy, and had her torpedoes had been working, the Royal Navy would most likely have had to divert more resources into hunting down and killing the San Luis if she had successful sunk HMS Brilliant or Yarmouth.
Long story short, never underestimate an SSK, or you'll find yourself full of holes.
