Under nitrogen plasma immersion ion implantation, the energy density per pulse EPulse/A was found to be proportional to the amount of the N-expanded austenite phase (γN) in a superaustenitic stainless steel, as previously seen for an austenitic-ferritic alloy. Since γN occurs in a range of stoichiometries, the parameter was also contrasted with the ion fluence Γ, which affects the retained dose. Γ varies inversely with EPulse/A and influences oppositely the properties of the modified surfaces. The temperature was the same among the treatments (320 °C) to rule out thermal diffusion as an extra variable. The increase of EPulse/A (for applied voltages 6.2–10.4 kV) produces thicker layers (1.4–2.2 µm), as an increase in lattice defects and stresses enhances N-diffusion, while high Γ values favor the N-saturation at the top surface for the opposite reason. The lowest Epulse/A (6.2–9.5 kV) result in brittle cases with a possible thin nitrides layer on top, whereas γN prevails in the ductile layer produced by the highest one (10.4 kV). The N-concentration governs the strength against plastic deformation: hardness varies up to 32% from the highest (6.2 kV) to the lowest Γ treatment (10.4 kV). In summary, the correlation of Epulse/A with Γ is indispensable for controlling properties of the modified surfaces for performance purposes.