"K2" balance is just the default value of K2, with 4 modules and little pollution (probably originally balanced around K2's ultra fast singularity beacons?)
"custom-vanilla" is based on the progression of vanilla, in which for every tier, the pollution per crafting speed increases, and 2 more module slots are available.
"custom-SE" is actually the only one I've played with a lot, it's faster because regardless of machine size, only one beacon can be used. Also more module slots would break special resource production too much due to the multi-step nature of it. SE 0.7 any day now...
I don't think I had more considerations than this when I made this mod, it's just some rough ideas that I kept just in case someone want the machine as it is and others want it to be more balanced.