On the FTB forums we have mostly been discussing simple continuous operation reactors. The most optimal of these reactor designs achieves an efficiency of 4x. These are all (in the terminology below) Mark I reactors.
The Mark II reactors - as I understand it - will all run for a single cycle, and then you will need to refuel them and replace used up components.
The Mark III and higher reactors will not run for a complete cycle. So you need to construct timing circuits (redstone?) that ensure that they are on for no more than some number of minutes, then have an off cycle to cool down. Mods like nuclear control if present in the pack have devices that can emit redstone signals at threshold temperatures IF the design uses hull heat - some component cooled reactors have zero hull heat until they fail catastrophically as the components burn up so those designs just require a well regulated timer.
These reactors have massive efficiency and output factors that mean that, even though they have an on/off cycle their averaged EU/t can exceed the simple Mark I reactors discussed on these forums.