Whilst I cannot offer any thoughts on the turbine speed, are you sure the turbine is being turned directly by the burned gas? I have understood that in a gas powered electricity generating unit (which is what I assume you're talking about) the gas is used to heat water or some other liquid which is then used to drive the turbine. I'm not an engineer so I bow to the far greater knowledge that others have and may offer you but I believe this is more efficient that driving the turbine by heated gas because of the energy that can be transported by a liquid for any given volume.