Are you basing this on NASA's Z-Pinch engine concept? http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120002875_2012002392.pdf If so, since that includes estimates for dry mass, wet mass, and the delta-v it's supposed to be capable of, I think one should be able to calculate its specific impulse. Running the numbers in the NASA link through the strout.net delta-v calculator seems to indicate a specific impulse of about 6650 seconds. Not sure how you'd go about calculating atmospheric isp or what the thrust should be though.
