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And I don't think blurred RT shadows are that bad. Have you experimented with shading rate? The shading rate of the surface receiving the shadow will influence the number of samples used to generate the directional shadow.

raytraced 20 blur shadows and a shading rate of ',2 and i still see a lot of noise. if you use a bright IBL then you can get away with it. but if you use dark shadows its very bad.