The whole netherrack -> lava thing came about before gregtech advanced pumps and has been nerfed quite a bit since it originally came out. While it is still technically energy positive it isn't as effective for power generation as it was before. As it is now it's more so you can burn netherrack into lava for centrifuging when you have an energy surplus, or for keeping a magmatic engine busy.
I just tested the vanilla hoppers with barrels and it does work and they are more than enough to keep up with a single magma crucible, you just have to be careful not to have redstone signals around them or they shut off.
Edit: For perspective, the crucible used to turn netherrack into a bucket of lava for only 4000 MJ, while in the 1.5.2 beta (and probably the 1.4.7 packs) they take 12,000 MJ per bucket, while a magmatic engine produces 18,000 MJ per bucket. So it went from a net gain of 14,000 MJ to 6,000 MJ over the course of its implementation.