I just checked using your example, and apm is active!
For an update, I added 3 more 2tb drives into my array (same brand, same specs, but the "green" label drives instead). I basically followed this guide:http://allintech.info/2008/06/mdadm-growing-raid5-array-ubuntu/
Basically you create new partitions on the drive the same size as the others, mark them as type raid, then add them into the array, grow the filesystem, then finally resize the filesystem to use the new space.
I did not have to unmount the filesystem, resize2fs now works with online drives. It took about 50 hours to grow the filesystem (due to its large size), and apparently about 16 hours to resize2fs. Now I have 6 2tb drives in total, for a total usable space of 9.3TB. resize2fs did see them as 4K block sizes.
When it was done, read speeds were still ~160MB/s as before, but write speeds were abysmally slow (12.5MB/s). I edited the tune_raid.sh script to turn off apm on these new drives, but it looks like they don't support this command (they give an error only on the new drives). To get proper write speeds all I had to do was reboot the box, and then read speeds also went up. Just re-running the tune_raid.sh didn't seem to help for some reason.
Now with 6 drives in raid5 I get:
Read speed: 330MB/s
Write speed: 125MB/s
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