I would be very surprised if a noise cancelling headset caused more problems than a standard set; I thought they just used a signal from both sides of the mic 180 deg out of phase to get rid of the cockpit noise but not affect the speech (and other noises!) coming from your mouth.
Basically if it sounds quieter in your earphones, it is quieter.
As always I'm ready to stand corrected, but it sounds like those studies might be of the kind that spend years working out things like 'eating too much makes you fat!'