I keep hearing that current Canon lenses are not 'good enough' for newer/better sensors. I would really appreciate a lesson on how this works.
Who did you hear it from, and how did they take their measurements of the lens MTF? Most of the online lens testers, including DXO test a lens on a camera, and in every case I've seen, the camera sensor has been the limiting factor.
Roger Cicala at Lens Rentals has the capability to test a lens by itself (No camera attached), and has posted a couple of actual lens MTF values for some conditions. A lens like the 24-70 f/2.8 L MK II or the 70-200mm f/2.8 MK II is supurb. Easily capable of handling high MP sensors. The Canon lenses may be slightly better than the Nikon equivalent, but they are both fantastic lenses.
Here is some more reading on the subject. Jrista throws out a lot of terms that you might not understand, this will help understand what they mean, as well as the other factors that are involved in grading a lens. http://www.cambridgeincolour.com/tutorials/lens-quality-mtf-resolution.htm