I don't know how well documented this theory is, but simply theorising about it, I would say that it doesn't sound very likely, first of all, the MiG-25 and the F-15 are two drastically different airplanes,just because the US thought it to be a super fighter, it never meant that the russians intended it to be anything else than a long range, high speed interceptor. Actually I think the MiG-25/31 may be the best interceptor airplanes ever built. They had a very different goal and mission than what the F-15 has or will have. as for the A-5 it was meant to be a bomber.