MOX reactors were initially buggy and got fixed in build #288-#297. Previously you got a lot more additional multipliers out of the reactor's absolute heat value and out of how close the fuel rods were to the upper left corner of the reactor GUI, neither of which was ever intended. Now you only get the one intended multiplier, which is percentage of maximum heat, and it scales up to x5 at 99.9% hull temperature. If you dare go so high, that is.
For the same reason, you can ditch all your heat plating and just stuff your fuel rods into a 0-chamber reactor - it makes no difference anymore in your case.
And no, transformers do not output per side. They will always output a quarter of their rated power in step-down mode, and if you connect to multiple sides, it will simply be split between them. You do not get more than 2048 EU/t out of an EV transformer in step-down mode, no matter what you do.
In other words, high EU output builds are completely worthless because transformers truncate any additional EU's. Gotcha.
So, no builds which can produce more than 8k EU/t are worth bothering with. Good to know. Kinda kicks my plans in the teeth, though. Imagine a Tower of Power with MOX reactors... such a sight shall now never be.