Actually Tetra is pretty easy to find outside Extreme Hills and can also be found in Green Hills if playing with Extra Biomes XL. Also for your power needs Lignite is nice since it spawns in huge vein quantities and burns longer than Charcoal I think. I'm not on my server atm so going off of memory here so try Green Hills since it often spawns in villages along with Bauxite.
No, in 5.08 lignite burns 3/16 as long as coal/charcoal (or the same as planks) in a furnace, and gives 1/4 as much energy in a GT boiler. It's been a bit since I've looked at lignite in 5.09, but the changelog says it the burn value was rised 3x, which would make it burn 9/16 as long as charcoal in a furnace, and possibly 3/4 as much energy in a GT boiler.
Also, for finding tetrahedrite veins, you can also find them in the Nether (and it's fairly common for there to be Netherrack at layers 80-120 for it to generate in), and they're more common because there are fewer vein types that are valid for the Nether (although I discovered that Biomes O' Plenty (which adds more biomes to the Nether in 1.7.10 at least) somehow interferes with that and causes overworld vein types to generate in the Nether)