Whether you have a square or a circle, and the markings tell you f/1.3 or f/22, what matters is the actual quality and actual amount of light hitting the sensor.
The quality of the glass is harder to keep up as you go out to the edges, right? So the ideal iris will be round, letting you select a progressively better portion of the glass.
Also, personally, I loathe pentagons, octagons, what have you... it is a real pain to have to pick between a geometry class reminder and lower IQ. There are a lot of very intricate elements that go into lens design and performance, but the high blade count sounds lovely.
