There is NO arrow on the market heavy enough for this to happen. Think about how loud a bow is with a light arrow. Sure if you had a 10 pound arrow you might see diminishing returns, might.
You are wrong and there have been some tests done in the past to prove it. If some of the "techs" jump in they will verify what I am telling you. Actually, you will start seeing deminishing returns at 700 grains or so. The tests gone be done by shooting arrows of different weight and differentiating at what points the arrows begin to gain and decrease more rapidly in speed over a given weight range!