For the first couple years of PoY, my schedule was "every two days" which meant I was always posting on at least one weekend day. It didn't matter about anything else, those were always the fewest readers. Through the week, the highest viewed days were always update days, but I never noticed a huge difference between MWF and Tues/Thurs. Now that I'm on MWF, which I chose only because it spaces out three comics per week as evenly as possible and avoids the weekends, my unique hits stat has a definable,if rising, pattern.
Updating on a Sunday isn't going to net you an appreciable amount of readers. I understand your logic, but it presumes that most people read webcomics every day. They don't. It turns out that most people read webcomics at work or at school, or at night when most comics update. Plus, when people read webcomics they tend to do it in bulk. They don't read one comic, they read a bunch of them at once, so being an outlier isn't necessarily a good thing. This isn't to say a Sunday comic will never have a bigger audience, readers will just read it on a different day, but that Sunday just isn't a day when people read them or go looking for them, and not because they aren't there.
Tuesday and Thursday comics are rarer update days than MWF but there are still a lot of them. If you want a lot of hits on the day you post, any weekday seems to be fine but weekends are going to be a bust. If you want people to be sitting at their computers, waiting for that moment when they check to see if the new comic is up. And that is what you want, you want people to be anticipating the new page, you want it to be a markable event that has them clicking their mouse eagerly. If their mindset is that they'll get around to it, y'know, whenever, then their excitement about your comic is low. Most of the bigger comics and the dailies update on their author's midnight, which is usually midnight EST or CST. It pays to be a part of that traffic.
The day that you update has some importance but the time you update has a lot of importance.