Between classes, I shelled out $5 to read the whole thing (abstract here), as they say.
The primary difficulty with this study is that it's not possible to extrapolate anything useful from it. To begin with, the title is "Are Tenure Track Professors Better Teachers?"; the study actually asks "Are Tenure Track Professors at Northwestern University Better Teachers at the Lower-Division Level?" (Answer: No.) I'm reminded of the joke about the book that starts out big in the main title, then turns into a hyper-specialized monograph after the colon--only here, there's no colon. As even the authors admit, "Northwestern University is one of the most selective and highly-ranked research universities in the world" (16), with predictable results both for the kind of students it serves (a mediocre student at NW may not be a mediocre student elsewhere) and the kind of non-TT faculty it hires. Northwestern is...Northwestern. One might expect similar outcomes from a study done at Harvard or Yale. Moreover, Northwestern's location not only makes it easy for the university to take its pick of Ph.D.s, ABDs, and MAs from other high-ranking research universities, like the University of Chicago, but also gives it access to a wide range of active professionals in other fields (business, the arts) who may adjunct as a sideline. It's not immediately clear to me that Rural Minor Branch Campus, lacking these amenities and with a very limited non-TT pool, will experience quite the same effects.
Of course, that being said, in one respect the results aren't surprising at all, for reasons that have nothing to do with the Platonic qualities of TT faculty as teachers. Teaching lower-division courses, especially large-scale lectures, is its own unique skill set. Pitching the subject properly (and making it attractive), organizing lectures for clarity, knowing what can't be included, speaking effectively to a more impersonal (because larger) audience, even changing how one uses gestures and body language...all of these things require practice. And if non-TT faculty are doing most of the lower-division teaching--and, at Northwestern, apparently often doing so for years--then it should shock no-one that they also wind up doing better at it than Professor X, who rotates into the lecture once every three or four semesters and otherwise hangs out with upper-division/graduate students. (Historical example: one of UCLA's acknowledged all-time great teachers was Albert Hoxie, a lecturer with an MA who specialized in the gigantic Western Civ survey. You can hear some of Hoxie's lectures here.) Cinderella's Fairy Godmother does not show up, chant "Bibbity-Bobbity-Boo," and magically equip either TT or non-TT faculty with lower-division instructional skills. At the same time, non-TT faculty at Northwestern are also not performing under the same conditions as non-TT faculty at, say, Underpaying Community College or Budget-Crunched Comprehensive, given far better pay and greater continuity of employment.