The magma crucibles run off of MJ and not EU. They use 20k MJ worth of energy for a bucket of lava, which can be used for only 18k MJ, so it's not self-sustaining. All the methods that convert EU to MJ come at a heavy loss, which makes, say, using an electric engine to run the crucible wouldn't be effective and you would lose more energy than you gain.
The problem with the steam turbine is that it's a massive investment and, with Gregtech, you'd lose a lot of potential eu. What I mean by this is Gregtech's matter fabricator takes 16m EU to create 1 UUM, and it takes 5 UUM for 2 Iron ore (which can be processed into 4 ingots rather easily). This is problematic because while the turbine recipe creates them two at a time, it takes around 99 steel ingots, or 49.5 ingots per turbine. Now, since my last calculations, the amount of steam used was doubled, but so was the output. Even so, over the average lifespan of 41 hours, it only creates 295,200,000 EU. This is at the new 320 steam/tick, or the equivalent of 64 MJ/t. this comes out as 188,928,000 MJ used to create that EU. However, for a fully self-reliant system, you would want to be able to replenish the turbines without having to worry about mining iron. However, each turbine roughly comes out to 792m EU to reproduce via UUM. Now yes, if you had a massive amount of iron you wanted to destroy, you could do that, but with thaumcraft you can always turn iron into other metals fairly easily, and there are many other uses you can use for iron. Basically, it's a massive investment that really doesn't help much. With the Thermal Generator running off of lava create by a magma crucible, for that same amount of MJ you can create 283,392,000 EU, or a 11,808,000 EU difference. So for 11.8m less EU, you can run a system that is fully self-reliant.
Edit: Another thing to note, a single Igenous extruder can quite easily fill a massive row (tried up to 18 before they started using less energy) of Magma Crucibles, and it would take 4 crucibles and 4 Thermal Generators to produce just shy of the amount of power of a steam turbine. This makes the two systems about equal size as well.