Does KV in rc motors (RPM/Volt) tell us anything about the torque? Considering that KV is RPM/Volt, does it ever tell us anything about the torque? Wouldn't you need to be told the torque separate from the KV rating?  If torque can be derived from a KV rating then why is a lower KV rating considered more powerful?
 A: For a linear DC motor the "back emf constant" in units of volts/rad/sec is equivalent to the torque constant of the motor in units of N-m/Amp. But the manufacturers will usually specify the back emf constant in units of volts/RPM, so the numbers will differ.
Strictly speaking a brushless motor is no a linear DC motor, but rather a synchronous motor, and the equations of motion that model it are nonlinear, and much more complex than the linear DC motor. But when feedback is used to control the brushless motor, its behavior can approximate the linear DC motor and so sometimes they will specify an equivalent back-emf constant.
Judging by the units you write for "KV rating", I would suppose its the reciprocal of the back-emf constant. So a lower KV rating implies a higher back-emf constant or equivalently higher torque constant, thus a more powerful motor.
A: Yes, you can relate Kv constant to BLDC motor torque by a very simple f-la:
$$
\tau=8.3*\frac{I}{K_v}
$$
or
$$
K_t=\frac{3}{2}*\frac{1}{\sqrt3}*\frac{60}{2\pi}*\frac{1}{K_v}
$$

a thorough explanation can be found here
A: There is another constant used to describe electrical motors that quantifies the torque, the torque constant Kt
$$ K_t =\frac{\tau}{I} $$
where $\tau$ is the torque, and I is the current. 
Therefore, the torque a motor produces is $ \tau = K_t I $. The SI unit of Kt is the weber.
It turns out that:
$$ K_t = \frac{1}{K_v} $$
So a lower Kv implies a  higher Kt; a higher Kt means the motor will produce more torque from a given amount of current.
Also, a given motor can be wound with more turns of thinner wire (higher Kv), or fewer turns of thicker wire (lower Kv). Thicker wire can handle more current; a motor that can handle more current can create more torque without burning out.
A: All else the same, KV is inversely proportional to the stall torque of the motor. But, all else is seldom the same, and stall torque often isn't that useful a metric.
