Yes, obviously there is a considerably split in sample quality between different Korg instruments (maybe arrangers versus high-end synthesizers) and also between presets versus expansions. Those M3 expansions were taken from the OASYS, which had a 7,000� price tag... In these spheres, it is also insiginificant whether a sample is from 2014 or from 2005, even a sample from the late 1990s can be superior if it was produced thoroughly and with enough megabytes of data.
As you say, there is a problem if vibrato is sampled and there are not enough samples per octave. The vibrato speed varies terribly then. On the other hand, no artificial (edited) LFO vibrato can get as realistic as a sampled one. I hate edited vibrato, you immediately hear that it is not from a real instrument. So one would have to have samples for every single key, and obviously Korg didn't want to spend that much waveROM for saxophones in the PA2X.

Funnily, in the case of Yamaha it is partly vice versa: a lot of the Tyros voices are not part of the pro synthesizer Motif: it doesn't have SA2 voices. But as I said, on the other hand, the Tyros doesn't have the higher-quality pianos the Motif can load into its flash board.