I can't speak for any of these drivers, but personally I'd say my helmet is more of a problem for my peripheral vision compared to the body on my car.

Although I'm only a mini sprint running a midget chassis, I have a Maxim arm guard on and decent sized side visor. I don't notice much of a difference compared to when the right side was a lot more open. Except it blocks a lot more dirt hitting you, the sun and the bright track lights, having it more closed off. Also most containment seats the helmet visor is usually above the right side head rest.