So the backplane should be considered, but you can probably safely bet it won't burn 50W. Maybe 10W or so, probably less. I'd imagine you'll be running these all at 12V, so startup power is about 24 W per drive. 8 drives starting at the same time gives you 196W. Add 10W for the M1015... then you've got 94 W left before you hit 300. I'm willing to bet the processor and mobo will never burn that much combined, so you'll probably be fine, especially if the M1015 will enable staggered spinup. If you want to double check, turn on your system with no drives, or maybe half of the drives and use a power meter to see what the max power draw is and then estimate the maximum with all of the drives.
EDIT: Wait, actually... strike that... drives use both 12V and 5V, so it looks like your startup power is more like 30W per drive. You could do the 300W supply only if spin-up was staggered by the M1015. Might be safer to go with something higher.
EDIT 2: Well, rather, some drives... my WD Reds only use 12V according to their datasheet