I've done some number crunching and I think I have your answer for you. These numbers assume that (a) you use the mining method in the accepted answer you link, (b) that the ores are uniformly distributed, and (c) that you're running a version of Minecraft between 1.6.0 and 1.7.3 (the chart you link hasn't been updated for 1.8 yet).

Mining for an hour with ~30 iron picks should yield you:

• 7,201 cobblestone
• 77 coal
• 308 redstone
• 46 iron
• 8 gold
• 8 diamond
• 37 lapis lazuli

Similarly, using ~6 diamond picks should get:

• 9,166 cobblestone
• 98 coal
• 392 redstone
• 59 iron
• 10 gold
• 10 diamond
• 47 lapis lazuli

As you can see, the iron and diamond yields are far below what goes into the picks. In other worlds, through solid stone, you operate at a loss. Caves make an enormous difference when it comes to mining efficiency.

(All this, of course, assumes my math is correct. Feel free to double-check my work in the spreadsheet.)

There is none

There is a big stonking problem with the double-extender. It doesn't work vertically. At least not, if you want multiple of them after one another.

In order to line up the pistons, only one side may be used to power blocks adjacent to the pistons. So, let's look at what our redstone has to do when, and where it can do it.

For a down-pushing double-extender to work, we need to take a look at the following schematic:

In order to "close the gate", we need to extend the upper piston first, by powering the top block:

Then, we extend the lower piston, by powering the bottom block:

With this, the double-extender is fully extended. Now we need to pull it back in. For that, we first retract the lower piston by powering down the bottom block:

Now, retract the upper piston by powering down the top block:

Ooops! Now we have a floating block.

To fix that, we need to extend and retract the lower piston again, by powering the middle block:

Good, now we have the theory down.

In practice, this cannot be done on just one side (which is the requirement for having multiple of them in a line). It is impossible to power all three layers individually at these distinct times, because redstone takes up a vertical air block.

A condensed one-sided Double Extender only works horizontally (as taken from Minecraftwiki.net:

Vertical double-extenders only work with powering from multiple sides. For that purpose, I have also built an upside-down version of this vertical double-extender (again) from Minecraftwiki.net: