If your turbine is high time, the nozzle could be the issue. Each hole in the nozzle of the turbine is like a small venturi; if they erode, your turbine loses performance, and will not cool at the same rate. This allows to much flow across the turbine wheel, and not the compressor of the turbine.
If your turbine is low time, shimming between the nozzle and the turbine wheel can often times be the culpurit. Either way, just because your turbine is spinning fine, and sucking, fine, it could be low in performance.
The only real way to tell if your turbine is performing at an optimum level, you would need to perform a functional test on it. We check ours for turbine inlet air pressures and temps, vs turbine outlet pressures and temps, fan inlet and outlet pressures and temps, and so on and so on...