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?