Sorry, not meaning to pick on Vmax, he seems a nice enough guy, I don't know him from Adam. I just have seen this before. Somebody makes a speed claim that is CATEGORICALLY higher than any previous record...chances are it is a timing error.
Like the 287mph Sig Wonder.
You could hit 355 while still attached to the bungee, probably.
It reminds me of a story I once heard about a bunch of aeronautical engineers who used to have lunch hour races in the big Boeing hangar in Wichita. They would string wires across the big room, and then fly CO2 capsule powered rockets down the wire on hooks. They made up a rig using car valve springs punching a nail into the capsule end to set them off. They got really competitive, doing all sorts of aerodynamic refinements and smooth finishes and stuff to win races. Heating up the capsules and stuff, all sorts of tricks. One day somebody accidentally launched one with an empty capsule and clocked the same speed, which made them realize it was the valve-spring launcher doing the pushing all along!

I THINK that has something to do with this? Maybe not.
Anyway...ask MEC how hard it was to go up to 300mph, and how hard it is to make small gains at such high speeds. I'm not accusing anyone of lying...just think they have a really inaccurate measurement there.