Just to touch on a few of the discussion points, as others have done. The OP poses a good question, and the basic differences in build spec that Tom, Smokey, Mark and others have outlined are the major factors. My Super Six was built in the same way...basically a "Rocketized" Six, in much the same way that an HRII is a mutant 4 (Mutant Rocket Drivers...I kinda like that for a patch...whaddya think).
Bottom line is that there are significant differences that justify the higher Vne...and significant testing in the field to support it. As Smokey will tell ya, to take a 4 to Rocket speeds, without the design changes, would not be recommended...and he has a well known flutter story to tell as well.
Mark's flutter in Reno was in part a result of a start that was quite a bit faster than briefed. The high drop-off speed helped Mark get past his Vne, and he did a great job handling the situation. The flutter and failure of the trim tab cable and the trim tab itself was also attributed to a trim tab actuator horn that was not within Mark's specs. When we rebuilt the tail in my hangar, we incorporated some improvements in the design of the tab and the surrounding structure. I will be copying that on my S6 as well.
Our class procedures at Reno have been modified as a result of that event, and we now drop off metal aircraft at slower speeds, at a lower altitude (flatter trajectory), and later (closer to the start pylon), so that there is less time in the chute in a full power descent. That keeps the metal guys safe, and we actually use a flatter trajectory, albeit at a higher speed, in the Gold races, to allow the turbocharged animals to be further up on the power at the release...in this case to help prevent prop overspeeds.
We really do emphasize safety at Reno, and as Axel said, we must certify that we have tested to a speed that is 105% of what we can qualify at. We are not at all flying in uncharted territory...just want folks to know we don't take safety lightly at all there! All out speed when down on the course and level is not our most critical point, in terms of flutter...its in the chute...and we are taking good safety measures there. Bill Beaton qualified at 281 mph in his highly modified HRII, but was very careful about his speed in the chute. Its not dissimilar to what Mark said about lowering the nose at 12.5 and leaving the black knob far forward...Bill knew better.
Finally, Dave Anders' 4 is anything but stock, as others have said. Its a work of science and art...and we may get the chance to see it in Reno this year! I sure hope so!
Cheers, Bob