I don't believe it's possible to make more money with the double and quadripple Nefastium then with the single. I have 6 single Nefastiums and the rest of my generator room is completely filled with Capacitators, vents and Heat exchangers. I am making a cool 2.6T per tick now.
For everyone intersted in how they determine how many AI you earn. The formula seems to be the same as that of cookieclicker:
(SQRT(1+8*(N/10^12))-1)/2