I've done some tests with XP18000 the last few days,
The xp18000 will charge the M8 for about 3 times under these conditions:
Using 9-12v output (10.5v actual output)
Charge when battery light at the back starts flashing
M8 was not turn on to charge the Ipod, But I did use the USB output on XP18000 to charge my ipod classic at the same time.
It will take take 4 hours to fully charge the M8.