Are you familiar with torque vs horsepower curves for car engines?
Here is a simple explanation of gearing down (shifting gears) to keep the engine at the top of it's horsepower curve (not torque curve).
http://www.allpar.com/eek/hp-vs-torque.html
You don't want to be at the top of the torque curve for your turbine, you want the top of the horsepower (in this case electric output) curve for you generator head. You are running steady state, so you should want the max power out, not the maximum theoretical braking torque. In your case 250 rpm is apparently the max power point for the generator head.
You need to measure the brake hp at the output shaft of your gearbox coupled to the turbine, when the output shaft is turning 250 rpm.
http://www.land-and-sea.com/dyno/dyno.htm The link goes to a business that has portable dynamometers. Knowing the brake hp at 250 rpm, you will know the maximum power available to run the generator head, and if it is enough to produce the electricity you have been told it will.
