Vmax...it seems to me YOU don't get it. A hand-held GPS just does not seem to be the appropriate tool for this job, and your figures come up 50mph higher than anybody else ever recorded. Guess what? You are not the first person who came up with readings that were 50mph higher than everybody else...and turned out to be false.
Go do some new speed runs with some proper measuring equipment and see what it says. Convince us all. I would be thrilled to be wrong, to find you were actually doing 355.