Turbocompressor (mechanical drive) gas turbine Power measurment

I want to measure the power of a two-shaft 25 MW gas turbine (Siemens SGT-600) and I think the best way to do this is by using a torque meter. I just wanted to know if anyone has any experience in attaching a torque meter to the shaft of the gas turbine and its obligations and problems. For example one of the most challenging issues is the shaft balance. Also, I would appreciate it If you can introduce some vendors who can do this job for me.