The biggest win is in the two huge fugues. With the slower tempo, there's more room for them to ratchet up the energy. In one recording I used to have, the "Amens" near the end of "In gloria Dei patris" sounded screechy. The "O miserere" passage on this recording is the most dramatic rendering of it I've heard.
There are a couple of places where the slower pace doesn't work very well. The "Et resurrexit" (pardon any errors in my Latin spelling) really needs to go fast, and the trumpet calls which interrupt the "Dona nobis pacem" were a bit plodding. But these are greatly outnumbered by the passages where Davis's choice of tempos makes it work.
I don't know if this recording is still available, but I'd strongly recommend it.