Whoa, 400? Best I've seen is around 100. (My own design, recharges 104 isotope cells per uranium cell:
link.)
I am trying to see just how much energy you can get out of a single piece of uranium. We know that uranium and thorium cells produce 1M EU * reactor efficiency, plutonium cells 4M EU * efficiency. Ignoring efficiency for a while (let's assume you can run an eff7 reactor and automate its cooling), this is what I got:
1 uranium --> uranium ingot --> 1 uranium cell = 1M EU * efficiency
16 uranium --> 16 uranium dust --> centrifuge into 16 uranium, 1 plutonium, and 4 thorium cells --> 24M EU/eff, that is 1.5MEU/eff per uranium
1 uranium --> uranium ingot --> 8 depleted cells --> centrifuge into 4 thorium cells = 4M EU/eff
1 uranium --> uranium ingot --> 8 depleted cells --> 8 uranium cells (8/104 = 0.077 uranium cells used to enrich) = 7.923M EU/eff
1 uranium --> 8 depleted cells --> 8 re-enriched cells (0.077 uranium cells used) --> centrifuge into 3 delpleted, 1 plutonium and 4 thorium cells. Doing a series sum to account for re-enriching the new depleted cells yields (trust me on this, I can provide the math) 11.8144 EU/eff per unit of uranium.
Assuming an eff7 reactor, and neglecting centrifuge EU costs and cost of copper and tin, the last choice gives 82.7M EU for each unit of uranium. Quite an improvement over the 1M from just canning it and tossing it in a nuker.
Turning uranium into plutonium with UU-matter requires 16,666,667 EU, but yields an extra 3M EU/eff. Therefore it is worth it if your reactor's efficiency is at least 5.55.
(If you are cheating and using IC2 massfab with GregTech, the conversion is *always* worth doing, no matter your reactor's efficiency.)
I have no idea how breeding cells with thorium or plutonium works. The GT wiki doesn't say anything about it, and the planner shows the same results for both as for uranium. If anyone can fill me in on the mechanics, I can take that into account as well.