Initial setup. Four quarries total, two steam, two combustion, each being run by a different number of engines.
http://i112.photobucket.com/albums/n...-18_152913.png
All quarries were activated by the same lever, and left to run overnight, plus a little into the day. Should work out to around ten minutes, but I didn't time it. I went off this instead:
http://i112.photobucket.com/albums/n...-18_154036.png
That quarry was run by a single steam engine. It dug down exactly two layers in the time it had. This one become my 'control' quarry, as running with a single steam engine produces delays and is therefore suboptimal. This one, with two steam engines, is actually decent:
http://i112.photobucket.com/albums/n...-18_154145.png
and this one, for comparison, was one combustion engine:
http://i112.photobucket.com/albums/n...-18_154230.png
As you can see, two steam engines are roughly equivalent to a single combustion engine. Those quarries dug down the exact same number of levels. They might be a few blocks off of each other, but the difference is negligible. The last quarry was run with two combustion engines:
http://i112.photobucket.com/albums/n...-18_154316.png
It dug down a whole 6 levels, putting two combustion engines at only 1.5 times as efficient as a single engine. Clearly suboptimal. Faster, yes, but it uses a great deal more resources to run. After that, I got a little bit silly and ran a 5th quarry for about the same amount of time as I ran the others.
powered by this:
http://i112.photobucket.com/albums/n...-18_160034.png
The end result was what you see here:
http://i112.photobucket.com/albums/n...-18_160026.png
The completely ludicrous combination of five combustion engines managed to dig down a mighty ten levels. The boom was moving incredibly quickly, so I don't think quarries really have a max speed beyond "How many engines can you physically attach to this thing?" However! Each engine here was actually half as efficient as it would have been running singly.
Therefore, I conclude that the best setup for efficiency is two steam engines, or one combustion engine. More engines will take less time, but are wasteful of resources.