So, I have had some disturbances in my crunching schedule, but I've been able to gather some more intel on ARM performance and efficiency.
1. HTC One M7
Tested in 12-16 hour/day sessions, since this is my phone and I have to use it to make calls occasionally.
Snapdragon 600(4 cores, 1.7GHz) | ~700PPD | 5W | 140PPD/W
2. Samsung Galaxy Note 3 Neo Duos
Had this puppy for 3 days. Ran it almost non-stop, but only with 3 cores active.
Snapdragon 400 (4 cores, 1.6GHz) | ~900PPD | 5W | 180PPD/W
3. Cubietruck (CB3)
This one is my lab rat. It went through so much torture, I'm surprised it is still alive. Installed Android 4.2.2, BOINC and SSH server. Ran 24/7 for the past 3 days with 10% overclock, but now it is down for maintenance and relocation.
Allwinner A20 (2 cores, 1.1GHz) | ~255PPD | 3.5W | 72PPD/W
Horrible performance, efficiency is very close to x86 systems, because it has too many power-hungry peripherals. If used with a SATA HDD, then it will consume over 6W and reduce PPW to 42.
Conclusion:
If you have an average family of 4 members and each one of you has an average quad-core smartphone, you can get a combined performance of 2000+ PPD by simply leaving your phones charging overnight (equivalent of a dedicated Celeron G18xx cruncher). If you have tablets, set-top boxes and other insignificant Android-powered devices, you could probably double or triple that number.
The only assumed downfall of crunching on the phone is that it may reduce its battery life due to extended heat exposure. While BOINC suspends computation at 40°C, I'm pretty sure that battery temperature sensors are unreliable and in some devices may not work at all (e.g. generic chinese smartphones, Samsung, Lenovo and majority of ASUS devices). Samsung Galaxy Note was the only device that did not heat up that much. My HTC One was so toasty, I could just flip it upside down and use as a coffee cup warmer.