You are correct. A number of the 1990's gas power stations actually struggled hitting even 35%. Gas was dirt cheap and the owners were after a fast return. Spending money on energy efficiency just wasn't on the agenda.
The size of the boiler/turbine unit "generator" makes absolutely no difference to it's efficiency. For a long, long time 120MW units built in the 50's were (marginally) 2nd only to the 660MW units built in the early 80's and notably above the majority of 500MW units built in the 60's. Design and operating regime pay a much bigger part than sheer size. There is no need to stick you neck out "steam turbine stations" by which I *think* you might mean oil or coal fired generation in the UK has an absolute ceiling at around 38%, basic thermodynamics won't allow any more. More efficient heat recovery, use of process heat, and the use of low pressure turbine blade materials that don't get mashed to bits by wet steam were the big changes when gas fired generation appeared.
They will.
There is no theroetical limit apart from the difference between the actual working fluid temperatures and the exhaust temperatures. With condensing turbines and superheated front ends you can get a lot higher than that - at a cost in complexity and of course corrosion issues.
If you tap off the low grade heat for heating buildings etc, of course you can do even better.
More efficient heat recovery,
Gas plants are built for cheap, not for efficiency.
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