I've found that pumps pump in the reverse of the order they build, so a stack built from the bottom up will pump upwards one Z-level per step, but a pump stack built from the top down will move whatever is at the bottom to the top in one step. the pumps end up feeding whatever they are pumping to the next pump's input right before the next pump pumps.
The real question is: if a chunk of magma is moved up in a single round, is temperature recalculated for each Z-level the magma occupies, or does it only calculate temperature for the starting and ending tiles?
If temperature is calculated using the state of game determined at the end of each tick, then yes, this makes sense. Going out on a limb, lets assume Toady programmed the game to have every object do it's thing, one turn at a time, then when everything has moved, other calculations are done, such as temperature, per each square on the map, and then after all this is done, it is synchronized into a 'tick'.
Some implications if this is true, aside from a pump stack built from the top down 'instantly' transporting magma to the top in 1 tick (with no extra temp recalculation), is that if you place, say, a noble, in one of the input/output chambers somewhere in the pump stack, he won't burn to death, despite the fact that he HAD to have been immersed in magma at some point during tick generation (otherwise, the magma could never have reached the top: it still had to go through the stack).
Edit: Just thought of this...I'm a bit rusty on my DF pump logic. If it worked like you said, and magma was pumped from bottom to top in 1 step, what about the next step? Does it 'push' the 5/7 or 6/7 or whatever stack which is still sitting next to the pump out of the way? If it doesn't, won't that eventually cause each input/output chamber to fill?