JSF having DSI has nothing to do with stealth.... it was developed to ease some restriction on turning for the air flow... having a bump and dump are part of the design and is not the source for stealth design... imagine B2 bomber doesnt have it...
Intakes are a major consideration in stealth design due to the fact that they protrude awkwardly (variable geometry) and are a major source of feedback to radar signals. The DSI not only reduces this feedback, but also allows better air flow. So the DSI inlets do help reduce the overall RCS, however just by having DSI does not make an aircraft stealth, but it definitely adds a stealth attribute to the aircraft.