Nope. The diffraction effects correlate with f/number, not with iris diaphagm diameter. That's because the distance between the iris diaphragm and focal plane also matters for diffraction. To expand on your example, 300mm f/11 has an iris diameter twice as large as 150mm f/11, which you are thinking would decrease the effect of diffraction, but at 300mm that iris diaphragm is also twice as far from the focal plane, which increases the effect of diffraction. So, iris diaphragm diameter and focal length effectively cancel each other out, and the f-number ratio is the determining factor.