Humm . . . I wouldn't be so quick to suggest that his confessions are throwout due to torture.
No doubt the CIA will have created an interrogation plan that checks for false positives. For example, drilling Khalid Sheikh Mohammed on actions where the CIA knows he has no involvement. Or inventing imaginary people, places, acts and money transfers and interrogating him, maybe for months, on events that they know did not take place. All this just to secure an correct answer to a question that is buried right in the middle of the interrogative procedure.
Believe me, after what I know of NKVD/KGB interrogators, I am willing to bet that the CIA did not give Khalid Sheik Mohammed a chance to tell any lies at all - even when under water.