Best Evolving System Link + core combinations for training?

100% spawn core is pretty rare from the mobs, I've gotten one so far. I've gotten the unique core unlocked against my better judgement and am capable of a 150% spawn boost with the unique stacking with the normal boosting core.

I wish you could buy more unique cores...has anyone found any as drops?

Yeah I posted in another thread of one dropping (that I couldn't pick up because I had full storage Q_________Q, but I've found several others since then.