If I remember right, the Kneissl head size number worked like this : a "35" was 35% larger than the standard 70 sq inches, about 94 to 95 sq in, so it might qualify as a mid plus today.
My Kneissl days were in the mid-80s, when they used "Big", "Mid", "Twin", and "Pro" instead of 40, 30, 20, and 10, respectively. Only the Big would qualify as a mid plus by current standards. The stiffest Kneissl line, by far, was the White Star line, and though I do not have an RA rating for any of those frames, I can say that if the White Star Big is not what you are looking for — and it probably isn't, being a bit flexible by White Star standards — then none of the mid-80s Kneissls are.
I played with a White Star Big for a year, incidentally, but I never really liked it, and I eventually went back to the Prince frames that dominated the era. As Flanker2000fr pointed out, even the stiffer White Stars were not nearly as stiff as the Adidas frames they inspired, and the problem for me may have been that flexible WS Big upper hoop. The WS Big's 1985 paint job
was beautiful, though.
Flanker2000fr is also right that the Red Star line contained boron, and the general rule in the 80s was that frames containing boron were the stiffest available, but the Kneissl Red Stars were an exception to the rule. They were noodles.
Here's a good thread on those sticks.