It looks like I misread the card. Since it doesn't count itself it would be (X)(X)/(4X).
Mitosis would cost 5+1(duo)+2(extra card)
So with your estimates it would be:
No support: 3*3/4*3=3/4=0.75
Mitosis: 6*6/(4*6+8)=36/40=0.90
These are fairly close.
Just thought I should point out that the top and bottom row hold 8 creatures unlike the middle which holds 7.
So for those we get
Mitosis: 7*6/(4*7+8) = 1.16
... Not a huge difference, but it does extend past the 1 to 1 threshold. Of course, that requires extending your creatures to the top and bottom rows which is a tricky feat in and of itself.
Still, the rank limitation and cost should be sufficient to limit abuse with fractal and mitosis...
It occurs to me that flooding is a potent counter... On the other hand, if this card could survive flooding, the combo would be very vicious. Just thought I'd throw that out there for anyone looking for a card idea.