I ran a new 12x1 D5 in my truck(I know not the same, but hey I ran it)..
From what I can say of the motor, it's got a fair bit of bottom end, massive mid-range, and quite a bit of top end compared to my D4 12x2..
As far as efficiency, well, the biggest thing on efficiency is the driver, I slightly overgear my truck and don't use full throttle and am very smooth, which since you run 1/12 I'm sure you drive pretty much the same way... Compared to the forementioned 12x2 I did lose alittle run time, but the D5 was pretty cool as far as temp..
But the motor totally RIPPED.. I used to run a 12x2 Sonic 2(Reedy Big Comm) because of the massive torque it produced, and I think the D5 is right there with it..
I'm actually taking out the motor this weekend, and putting the 12x2 D4 back in, the D5 was just too much power( first time I've EVER said that
)