Yes it could still be the tensioner but when I went down a similar route with my mk4 (tensioner was flapping about, especially with the engine under load like when on full lock for example) I replaced the tensioner thinking it was that. Changed it and all was good for about a day haha. So did some research which pointed to the alternator pulley. They have a freewheel clutch mechanism in them, which dampens the ocillations of the engine. When these seize up, it causes the tensioner to waggle as the belt is speeding up and slowing down (exaggerated).
You can test by taking the tension off the belt, slide an Allen key or small screwdriver through the hole in the tensioner which holds it back, slip the belt off the alternator pulley, then with your hand spin the pulley without letting go and jerk it back the opposite way. You should be able to make it freewheel. I suggest wearing gloves if you do try it so the ribs on the pulley don't slice your hand if it spins.
