NASA is testing hydroxylammonium nitrate in space which is expected to perform 50% better than standard propellants and can be allowed to freeze (hydrazine must be kept liquid). LMP-103S has been tested gets about 30%.
used in Why would it be “less bad” for hydroxylammonium nitrate monopropellant to freeze than for hydrazine? and found in this answer to *Is the EU really banning “toxic propellants” in 2020? How is that going to work? * raised a question about how a monopropellant could be 50% better than hydrazine.
The reply comment links to GPIM AF-M315E Propulsion System which explains that the performance metric is the product of density and Isp.
Question: When would the density Isp product be an important performance metric of a propellant?