If you are using a U360 for both 4-way and 8-way games then you are going to need either a round or octagonal restrictor. I tried both and ended up going with the round restrictor because it just felt smoother and played very well indeed. But of course you lose that 4-way gate feel. Depends how much of a purist you are.
However... I am currently changing the control panel so that it plays only 4-way games using a reproduction Nintendo Donkey Kong joystick from Mike's Arcade. I wanted the bartop to have the original joystick look and feel of DK, which is the purist in me coming out. I only decided to do this since I finished my latest project wich plays most all games in Mame so I don't need to have the same wide choice on the DK bartop. On my bartop I only had room for two action buttons and I am even dropping that down to one now so that it looks more DK authentic. Not sure if there many 4-way games that used more than one button anyway.

I had the exact dilemma you are thinking about right now when I built the DK bartop. I knew it was for vertical games only but like everyone else that's ever built projects I wanted to try and cover as many games as possible. Sometimes that's not always the best way to go. You said yourself that Pacman and DK are your two most favourite games. So why not consider dropping the 8-way games and just have the cab play 4-ways? Then you get your favourite joystick and can play the other games on a separate PC. Having said that if you do decide to include 8-way games then the U360 is an excellent choice.