Easy to say, but can you back that up? Radar detection range is proportional to the fourth root of RCS. So on a first glance, a small relative decrease in RCS will not significantly impact radar detection range. I will come back to this.The difference between an F-18, Rafale to a J-15D is not likely to be that significant in the context of a powerful AESA.
However, when we consider jamming, as we should in any realistic scenario, the jammer power required to achieve the same effectiveness drops linearly with the reduction in RCS.
According to some debatable numbers (best that I have), the Superhornet has just 35% the RCS of J-15 (no data on J-15D). However, the Rafale-M has merely 5% the RCS of J-15 vulgaris in S-band and higher bands!
Therefore, if we assume equally effective jammers and radars on both sides, the Rafale-M will be detected at 20 times lower range than the J-15 would. I would call this significant.
Finally, in absence of jamming, again assuming equal radar performance, the Rafale-M would be detected at about 0.47 the range that the J-15 would.