Even if it isn't, I did the number crunching a while ago when this same question was asked.
Turns out, Magma Crucible-ing lava from a piece of cobble and direct producing smoothstone in the extruder is more MJ effective by a HUGE margin.
So if you want a machine that just chugs and produces smoothstone forever, I'd recommend that, but you'll need an aqueous accumulator to feed the extruder (you can actually feed some of the smoothstone back into the crucible from the extruder if you want to save space, still more space-efficient)
Powered Furnace: 160 per smelt
160 x 500 = 80000
How much to make 500 smoothstone? A bucket of lava.
How much to make it using cobble/smoothstone? 20000, and 14000 if you're using netherrack instead.
So basically you're using 4x as much energy by smelting cobble.