I've read and re-read the IC2 nuclear reactor, liquid heat exchanger, and steam turbine pages, and the Gregtech large heat exchanger page. I think I've figured out what's wrong with those pages, but haven't tested it in Creative yet.
The IC2 liquid heat exchanger page says "hot liquid" (hot coolant or lava) is 200 heat units / L. Then it says 1000 L lava is 20,000 HU. So it's actually 20 HU / L.
The GUI and the simulator talk about "heat / second." They don't mean HU / second. They mean Hot Coolant / second. The actual HU output is 20x that much.
The IC2 steam generator converts 1 HU to 1 L steam, or 0.5 L of superheated steam. That's 1 L hot coolant (20 HU) = 20 L steam or 10 L superheated steam. The Gregtech Large Heat Exchanger produces 80 L and 40 L respectively, 4x as much. It's even more efficient with lava, producing 160 or 80, compared to the IC2 heat exchanger which treats lava and hot coolant the same.
Getting superheated steam out of a Gregtech large heat exchanger is much tougher than getting it from an IC2 steam generator. You need 4000 L/sec of coolant in the Gregtech exchanger. An IC2 steam generator has no minimum, you just set a slider.
A superheated steam / regular steam turbine series is 50% more efficient with either setup.
Single-block Gregtech steam turbines have poor efficiency, but you don't want to use them with a nuclear setup anyway. Among other things, they eat the steam without returning the distilled water you need for the heat exchanger.
Gregtech rotors increase steam -> EU efficiency, but the big step up is the massive increase in steam you get from a large heat exchanger.
Where an IC2 setup can get at most 0.75 EU/ tick from 1 "heat / sec" (1 L coolant / second, 20 HU / sec, 1 HU / tick), using superheated steam, Gregtech gives you 2 EU / tick with regular steam, and 3 EU/tick with superheated steam. With a 115% efficient medium sized Ultimet rotor and regular steam, that's 2.3 EU / tick. With a Large rotor it's 2.8 EU/tick, but that requires tungsten. Superheated setups require at least 4 reactors to run.