I have an octagon restrictor installed on my U360. At first I thought it would be helpful with 8-way games, but I realize that when I move to the diagonals, I'm not hitting the exact diags on the restrictor plate itself. I'm using the generic Ultramaps, so it still registers as a diagonal, my stick is just not landing in the crook of the plate.
I should also point out that I don't play a lot of Fighting games like SF2, so I don't mind not "feeling" the exact diagonal. Taking this into consideration, I think with my next order from Ultimarc, I'll be buying a round restrictor to try out as it seems to be the preferred one around here.
I will say that the octagon feels much better as an 8-way option then a square restrictor where the throw is shorter up/down & left/right, then diagonals (for example on the Ultimarc's E-Stik in 8-way mode).
Lastly, the octagon's opening is cut at an angle, so I guess there are two different throw distances you can have depending on which side is facing up when you install it. Hope this helps.