Minecraft – Multiple Lava Pumps In The Nether

I just went to the Nether and did some research with WorldEdit's //count.

Here's how lava is distributed in a 200x200 area:

LB: 0  0  0  0  31 65 30 22 18 19  30  20  20  18  17  18  15  14  9   7   1   168  382  1879  5594  6864  8200  10084  12393  14205  16219  18187  71  38  42  44  55  57  48  85  46  50  52  55  38
%:  0% 0% 0% 0% 0% 0% 0% 0% 0% 0%  0%  0%  0%  0%  0%  0%  0%  0%  0%  0%  0%   0%   1%   5%    14%   17%   20%   25%    31%    36%    41%    45%   0%  0%  0%  0%  0%  0%  0%  0%  0%  0%  0%  0%  0%
Y: 0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21   22   23    24    25    26    27     28     29     30     31     32  33  34  35  36  37  38  39  40  41  42  43  44  45

(LB - lava blocks in a 200x200 layer at the Y coordinate; % - percentage)

We can see that there's lava starting almost at the very bottom. But big amounts of lava start only from Y: 23, so that may be the answer to your question.

Based on the numbers yes the magmatic engines would be more efficient in terms of lava used per amount of digging done. However you wouldn't quite get the full 4MJ/t/engine at your quarry because you'll need some sort of power distribution system (e.g. redstone energy conduits) to connect up all the magmatic engines you'd be using, but the loss would be very minor.

The downside of fully powering a quarry with 100MJ/t is that it will run ridiculously fast and your sorting system will need to have been designed (or hurriedly retrofitted) to handle the huge influx of mined materials without clogging up.