Thank you for the input. Yes, I've found the 11x17 standard Itoyas to be a little too snug for some of my artwork. Which is the reason I searched and found the Profolio binders. The removable pages are a bit wider, lay more flat, a bit sturdier and have better clarity. The binder itself it a bit more rigid as well. Good to know there is an 12x18 standard. That might have solved some of my issues early on. But after finding the binders (which also allow me to easily rearrange things), I think I am just going to use the standard Itoyas for less expensive commissions and prints. It looks like, until they come out with a 12x18 binder, the best thing is to go for the 13x19 binder with mylar.
For those that do use mylar, do you tape the flap down? I find the flaps a little hard to work with without a backing board, as taping them down tends to create a little bulge. I have just been inserting the open mylar into the sleeves being that they are shy of 19" tall. I suppose, if I did switch to a 13x19, I could just trim off the flap. Just curious what everybody else is doing.