I changed the t-belt too and I got p1338 (cam position sensor...) The only thing you can do is to move the mark the other way (1/2 tooth advanced, 1/2 tooth retarded). Mine is <1/4 tooth same way you have it. Before it was the other way and less power. We got it as it is now, and it has much more power, although the p1338 did not go away. Since the cam might not be the problem, then the cam sensor could be, or the crank wheel could be off (because I believe it is taken off).
Hope it helps, please keep me updated, and hope it helps.