Personally, my ADHD is usually manifested internally, in terms of extensive rumination and daydreaming. I spend a lot of this time examining and questioning my beliefs over and over again, always searching for the deeper foundation, the stronger position, etc. So, perhaps for me at least, I think my ADHD has actually helped me become and stay rational. But I get the feeling that each person has their own flavour of ADHD, and I don't expect mine to be representative of others. Maybe for some who spend time ruminating like I do, spend it engrossing themselves deeper in fantasies of religion and 'spirituality'.
I had a friend who I considered "exactly like me, but completely opposite" in that (speaking with hindsight vision) he had rampant ADHD and was very intuitive. But where I focus my intuition on rationality, he focused his on his own ideas about spirituality. He believed he had healing energy coming out of his hands. He believed he could see and communicate with spirits. Spirit guides, the whole bit.
So, I'm as curious as you are. Probably you'd need a better cross-section of the ADHD world to get a good feel for how rational they can be. Us atheists tend toward the rational more so than the general population, IMO.