P3-SL? You are using an oldie but goodie. The horizontal dropouts should give you some more room to play with if you back it out. However, I find tire varies from brand to brand. So ask for brand if someone says 28mm fit.

From my memory, I would think you are pushing it at 26mm. These frames (cutouts and width of foils) were most likely designed around a 23mm wide tire. This is what I use on my P3-SL.

I have HED Jet + with GP2000II 23mm tires with no issues. The 23s stretch a lot but never rubbed and I’ve put about 2k miles on that setup.

I have used HED Ardennes 25mm wide (21mm internal) with 25mm tires - no issues.

It's a 58cm frame size, though.

Biggest problem I ever had was tires that became too tall and rubbed at the fork crown.

I ended up with some 22mm wide Chinese carbon clinchers. No clearance problems. Thanks for the replies everyone.