In my opinion Geothermals don't need any nerfing. Pumps should simply not work in the nether. At all. Just like water can't exist there we can find some 'reason' why a pump would not work (too hot perhaps) and this would solve the issue with 'free energy' that Nether-pumped lava creates.
I'd rather see the pumps range be drastically reduced, right now it empties an absolutely huge area, people can be running the pumps for months without having to move them.
If the range was reduced it would be a much less lucrative option since you'd have to move the setup every few days.
Ofc this would have to be made so that it still were able to extract the oil from oil wells propperly.
Back on topic, while it isn't a direct conversion, i find peat to be a much better conversion then biomass/biofuel, each peat is worth 5000EU in a generator and a peat farm will spew out those thing.
While it's most usable at small scales it does provide a quite considerable amount of EU in the early game, and peat is always nice to have.
