well, you know its really hard to say without actually adjusting boost on the dyno, but if I had to predict something (which is all anyone can do on a forum) I would say your nearing the limits of that turbo already.
The tq taking a turn down isnt necessarily always a sure sign that the turbo is out of puff, but it is a good indicator.
Her are a few graphs I have that I can guarantee the turbo is maxed out on. (I tuned these cars).
the blue line on this one is the tune the car came to me with and the red line is what I did for it. Same boost level, the power difference was all ecu mapping.
And now this is an example of a turbo that is not yet to its limit. I know it wasnt because hp and tq were gaining something crazy with every lb, but I didnt want to add more boost because it was a pump gas car.
So like I said, It isnt a dead give away that your turbo is out of breath, your ecu mapping could possibly be bad causing the tq to drop but assuming it is good, then your turbo is nearing its limits.