...we estimate that optimized SSFC systems using established lithium intercalation compounds could have energy densities of 300–500 Wh L−1 (specific energy 130–250 Wh kg−1), which would satisfy metrics considered necessary for widespread adoption of all-electric vehicles.[4] Further improvements would be possible by ‘dropping in’ higher-energy-density or lower-cost storage compounds in the SSFC platform as they are developed.
...we estimate that optimized SSFC systems using established lithium intercalation compounds could have energy densities of 300–500 Wh L−1 (specific energy 130–250 Wh kg−1), which would satisfy metrics considered necessary for widespread adoption of all-electric vehicles.[4] Further improvements would be possible by ‘dropping in’ higher-energy-density or lower-cost storage compounds in the SSFC platform as they are developed.
http://onlinelibrary.wiley.com/doi/10.1002/aenm.201100152/fu...
(linked from http://web.mit.edu/newsoffice/2011/flow-batteries-0606.html)
So 0.47-0.90 MJ/kg. Based on theoretical calculations -- they haven't actually engineered this.