The reviews I've seen is that the Twist can only put out 2.5 amps. So the most power you can get into 1.5 ohms is I*I*1.5= 2.5*2.5*1.5= 9.4 watts. This occurs at E=IR= 2.5 * 1.5= 3.75 V. Twisting above 3.75V will give no more power.
9.4 watts is above the 8 watts many consider good, so not bad.
The atomizer that will give you the most watts with a twist is R=4.8V/2.5A= 1.92 ohms. In this case you can get I*I*1.92 = 2.5*2.5*1.92 = 12 watts. This is not a whole lot higher than 9.4 watts, so 1.5 ohms isn't so bad.
In comparison, take a 2.5 ohm atomizer.In this case the maximum current is 4.8V/2.5= 1.92 amps and the device is not current limited. So the power is 1.92*1.92*2.5 = 9.2 watts, which is about the same as for the 1.5 ohm atomizer.
(1.92 keeps popping up everywhere above. This is just a coincidence !)
The exact results should be taken with a little grain of salt since the current limit may depend on the voltage, the current limit may not be EXACTLY 2.5 amps, etc.
However, the conclusion is that 1.5 ohms is not too bad.
I myself use 1.8 ohms, which is close to the optimum (1.92 ohms) for most power.
(11.25 watts vs. 12 watts max. at 1.92 ohms) Nobody sells a 1.92 ohm atomizer anyway !