Nice topic, spent a lot of time myself on these \'cube generators\' and ive accomplished the following.
Dimensions - Blocks - e/sec - e per block
3x3x3 - 15 - 2051.6 - 136.8
5x5x5 - 64 - 4992.8 - 78.01
7x7x7 - 167 - 26,789.5 - 160.4
This tells us to stay well clear of 5x5x5, you could have four 3x3x3 reactors for the same weight and have almost twice the power.
However cubes are still very inefficient, take a look at the following two reactors, they both produce a little over 26,000 e/sec.
These both have much smaller masses than a 7x7x7 cube yet produce the same power. I realise the point of cube reactors is to stay compact but if you take the reactor on the right for example it is 68 blocks lighter!! And i think it is already starting to look like a space ship.
My 7x7x7 cube above can sustain the cloak, jammer and power 6 thrusters, now think what we could use the 68 block weight saving for!
So in summary, whilst the cubes do look beautiful they are just too heavy i think.
If anyone can beat my cubes e/sec records id be eager to see them.
thanks for reading.