That is the set-up I use. Lava takes 20,000 MJ to make. And 1 bucket gives 18,000 MJ in a Magmatic Engine. So 90% conversion efficiency. Not really any worse than 2 Redstone Energy Cells. But yeah, you don't need to use Netherrack in a Magma Crucible. You can just use cobblestone more easily.
But yeah, a 7x7x6 railcraft tank full of lava stores 90 Million MJ.
With that much, you can pretty much run down the lava tank until it's 50% full, using BC Gates, then turn on your steam boiler to make more.
The nice thing about Lava, is it can run in a variety of engines / generators. And you can always siphon some off into a Portable Tank as a "battery" of sorts, for powering Quarries and the like. Much cheaper to make a Portable Tank than a new Redstone Cell. A tank stores 8 buckets, so that's about 160,000 MJ per tank, which requires 1 Tin, 4 Lead, and 4 Obsidian.
I think this is the new system I'm going to make. Thanks for the post. The only difference will be I'll have a string of 6 or so energy cells for portable energy. They're a pain to make but with quarried resources I don't worry a lot about how much they cost to make.