The mod is useful, but not as much as you'd think. I have ran a couple of tests with high loads and i came to a few conclusions.
The setup was comprised of Q6600 B3 on Vapochill LS, Seventeam 600EAZ (very strong +12V, integrated power meter) and an EVGA 680i SLI A1 with 4x2200UF low-ESR caps added on Vcore PWM +12V input and 4x 1500uF low-ESR caps added on the Vcore output; it also had Vcore mod and removed Ilimit, and i pushed the voltage up to 1.9V.
Most important thing is that the multimeter only shows the average voltage and you can't see the ripple and voltage spikes, wich usually are the cause of instability. The Vdroop mod only keeps the average voltage stable, but it actually makes the ripple and spikes worse; for these you would need to rework the LC part of the PWM with stronger coils and better caps, as well as increase the pulse frequency of the PWM (if the mosfets cand handle it) and even tweak the feedback response in a way if possible.

Pushing the voltage very hard made the CPU put big strain on the PWM and showed alot of it's weaknesses; the system drew close to 500W with only the CPU loaded (Orthos + 2x SPi 32M). Tunning for minimal vdroop at 1.45V I couldn't use more than 1.6V stable, the transient load spikes were absolutely awful. Permitting the vdroop to swing about 0.03V allowed 1.7V+ with reasonable stability and when closing in to 1.9V i had to have a droop of at least 0.05V. Remember this motherboard has double the filtering capacity on the output compared to stock ones, wich would probably act mush worse in the same conditions.

Hope this info helps someone, the EVGA 680i SLI is not the greatest tool for hardcore CPU clocks but it can be improved and become competitive with the right tweaks.