I can only back up what Paul said, we both were lucky enough to turn some laps with the new plane yesterday and it feels so silky smooth and solid it is hard to describe.
As far as how stiff the plane is I can't give you any scientific results just the usuall Joe Manor kinds of tests that you do when you visit him which go something like this: "Grab the wing and place the tip on the table now put your weight into it and see how little it flexes, now grab the aileron and try to twist it, hey grab the horizontal stabilizer and try to twist it, here let me get a hammer so we can beat on that thing...".
All of that being said the plane is so much stiffer than any production plane I have ever held I don't even know what to compare it too. We flew last week in some chopy Santa Ana conditions at Norco, where you could visibly see the wing on an MCT that flew flexing and we were all commenting on how when the D-80 flew it just looked super solid with no visable flexing. I am not saying this to take anything away from the Opus, because they are obviously a well proven design, but they don't even come close to being as stiff as a D-80.
One thing that should probably be mentioned is the wing on one of these planes is so thick that just based on it's mass it is going to be stiffer because of the increased cross section of all of the components that go into it, I guess what I am trying to say is they are really, really beefy.
Hanging out with Joe one thing becomes apparent pretty quickly and that is that he is really proud of this design, and he should be - the plane is truly a great design.
If you are on the fence about ordering one, just pony up the cash and do it!!! You won''t be sorry.