I think most of the suggestions you have received are for dealing with the opposite issue. If your edges are too long because they have stretched, then a shorter border and careful pinning/easing can often correct it. But if your center is too big, then your quilt is going to puff up in the middle. Definitely don't cut your border to this size, because you'll just compound the issue. I would go with cutting the borders the length of the edges (average the two edges, just to try to keep it somewhat square). But you are going to have a serious challenge to get the center to quilt flat.
If you are willing to do some disassembly to correct the root problem and make the center of the star lie flat, then you will get a much better result that way. What I suspect has happened (because it happened to me on one of my early star quilts) is not that your star stretched (although that may be part of it). I think that your diamonds were not true 45 degree points when you assembled the 8 star points. And then when you joined them in pairs, they were not 90 degree square. I would take apart the center of the star into the 4 quadrants that you assembled. Carefully lay a square ruler on the center point of each segement, and trim it to 90 degrees. Then re-assemble the top. You could possibly do this without disassembling the y-seams and setting squares/triangles if you are careful (so you have a star with an open center while you trim. Then resew the partial seams to put it all back together.