Forget the problems with the sound. Thought in the 30s, the sound is perfect and Heifetz's violin sounds with every light and torment that made him famous.
This is not a "philological" Mozart, obviously: some "rubato" where it wasn't even conceived, some Brahmsian phrasing. But in the end, this ultralyrical Mozart strikes you like an arrow directly to the heart of the music, the heart of the feeling.
Even comparing with Mullova or even Stern (a little more Mozartian...), these recordings, with their diversity, are perfect accounts. I recommend to start with the "Andante" of the 4th Concerto: everything is there, feeling, intensity, perfect bowing, virtuosism, and a tremendous sensibility.
The Mendelssohn is good, but Heifetz's best reading of the concerto is with Toscanini, also in Naxos.