I do use a lot of of sodium, but mostly for ore-washing and large batteries. Yeah, lithium's a lot better, but 512 volt batteries require so much material (32 each!) that I can't really afford anything but sodium. I'm not really making the large batteries for raw energy storage anyway, I'm usually doing it to put a clamp on how much current a given power line can supply. I can be sure that a 4-battery buffer is never going to supply more than 4 amps, so I can safely use 4 amp cable downstream of the buffer.
FWIW, lithium batteries are not the highest capacity batteries you can make at MV and HV: an advanced batpack (from IC2) stores 600k EU (compared to 400k for a medium lithium battery), an energypack stores 2M EU (compared to 1.6M for a large lithium battery), and a charging energy crystal stores 4M EU. I don't remember if I've tried putting a charging energy crystal in a battery buffer, but I have used energypacks that way.
For a 4 amp cable at HV or lower, you've also got the option of using the next-higher tier IC2 energy storage block with a GT transformer to step it down.