I made a MOX reactor that works pretty well... but outputs only a similar heat to the same reactor with quad cells. This does not make it more efficient to me.
In the classical sense, efficiency would be a measure of fuel used to total energy output. The easiest way to mathematically compare two reactors total running efficiency would be to take the total heat generated over a cycle, divided by the number of fuel rods in the reactor.
Of course, if uranium is relatively free, but your big expense is in reflectors or some consumable component, then that might be a more meaningful measure of efficiency.
Its really not clear from your post what you are asking...