Not sure how much the variable voltage heads play a part, I think it is more the sensitivity of the digital components to brown outs.
Can't remember where I saw the technical paper and details, but after firing, the (and can't remember if it is current) the drop of power is just below the required steady power many of the digital strobes require. Some may be better than others.
I have also see other people who have tried recycle tests with some of the Elinchroms and while they do o.k. at lower power, once they boost up closer to full power, after firing they will throw E4 errors
With my photogenics, they are designed to discharge when powered off or when the wattage is decreased, so what I would see with the 2500s would be a fire, half charge, discharge, then full charge and they would be ready... in short, they did not like the current drop.
My guess with the Elinchrom power and why it is more expensive, it has a larger and better power inverter as well as not sure if the added in some capacitors, or just maintain more current and thus have a lower drop from a firing