> At what outside temperature would you REALLY need the preheat tube left intack to make the stove work properely?
Well ... I have used an upright at -5 C, but that took some doing.
There is no simple answer to this. What happens is that as you draw fuel from the canister, it evaporates inside the canister and cools the remainder. The longer you have the stove running, the colder the canister gets - and you are planning on cooking for a number of people with it. I have had frost on the canister when the air temperature was at least +5 C - and the stove stopped. Evaporation! :-)
If you put the canister in a bowl of (liquid) water you are supplying heat to the canister, strange though that might seem. And you can add a few spoons of warm water to the bowl as you go, so you can keep going to -5 C if you try. But at -5 C the water in the bowl is going to start chilling down as well, by its own evaporation. Tricky.
I suggest you look carefully at what weight you might possibly save by chopping some of the tube out. It is not going to be more than a few grams. It may well be that having the option of using the canister inverted when it is cold may save you that much fuel weight in one night, just by making the stove so much easier to use.
Personally, I would take the WindPro just as it is, in the expectation of having to use the canister inverted on a night or two. Chopping it might save you 50 grams at the maximum, but at the expense of considerable complications, unreliability and worry for the whole group. Remember: 50 grams is 1/5 of a standard teacup of water - that's not a lot. There is something to be said for reliable gear. Myself, I would look for weight savings elsewhere.
Just my 2c.