What I have found in simulation is that the combined sum of inductance in the whole VIC must be accounted for in the resonant property of the WFC. Whether this is true in application I don't know, but in simulation, I achieve a 15kV charge on a 702 pF cap using a value of of 721.65 mH on the coils on the secondary core at 5kHz and a gate pulse of 75Hz. The L2 on the primary core likewise has a relatively low inductance. Higher inductances on the coil directly coupled to the primary core seem to produce lower voltages.
Ei: the resonant frequency for a 702pF cap with a 1.44 H coil is 5kHz. According to the simulation to get high voltage on the cap, the total inductance of all the coils (and using the mutual Inductance value from the primary core, not the L2's inductance value) must be equal to 1.44H. And the L2 main inductance must be lower to get a higher charge on the cap.
Keep in mind that I am using the format of 2 cores where the L2 is coupled to the primary and the L1 is coupled to the secondary.
I will add that I have not been able to confirm this in application yet. Awaiting some parts to fix my WFC.