I'm building a fleet of passenger cars and have noticed that the only passenger cars that can get around my layout without skipping the rails are my Kato cars.

But it's not just from the 11" radius's, the wheels on the Kato's seem to roll smoother in general...

So of course my question is:
Has anyone retrofitted some Kato trucks/couplers onto Con-Cor or Walthers passenger cars?

I got a nice fleet of Concor cars with microtrains couplers/trucks on them that are just terrible on my layout, and the Walthers cars can't even handle a 15% curve without derailing...

How about using metal wheels? Fox Valley, North West Short Line, all make wheel sets. Just make sure you get the right axle length.
You might be surprised on how well the metal wheels work.