[binarycoffee]

I guess you mean the power-to-thrust ratio, which indeed scales roughly with the specific impulse.

The thing is, pretty much every plasma propulsion device has the theoretical ability to throttle specific impulse. Gridded ion engines and Hall thrusters can for instance directly throttle specific impulse (Isp) by varying the discharge voltage. Other plasma thrusters can do this by other means like increasing RF heating, varying the magnetic field, etc.

Unfortunately, in real life the actual usable range of Isp always ends up much narrower that initially predicted because of a variety of factors that are difficult to predict theoretically (erosion and ionization efficiency in particular).

AFAIK, due to the shear size and complexity of Vasimr, its thrust has never been directly measured and we know very little about its actual performance. I would therefore wait a little and discard sensationalist articles until real, extensive testing has taken place.