I think for the biogas setup, it makes a lot more sense to plug in RTGs for an extra 64Hu/t that never runs out, or to use a non fluid reactor.
The thing with mox is, its efficiency is shot, it uses six time the U238 as enriched uranium, barring having run out of 235 alltogether, you ultimatley only end up with the *same* efficiency as enriched for fluid reactors, and you have to build CRCS setups to pull it off. It's not bad if you're looking for direct to EU setups (I've got that 1200eu/t one in creative, without a need for a whole biogas factory/distribution point), but with fluid you're pretty much capped on output anyway, so the best you can hope for is equal efficiency.
The only exception might be if you've been running so long all the U235 is gone.
Edit: actually, i take this back, with CRCS fluids you could push over 10kHu/t per tick the mox is actually on. Actually building that much in coolign towers is slightly absurd, but the efficiency is good enough to be competitive with other plutonium options.