Of the 1,700 sounds of the BK-9, 1,100 are identical to the BK-7m. The other 600 are a mixture of a few (22) superNatural sounds, maybe 200-300 from the SonicCell, largely identical to Fantom X, and some hundreds from various sources including a few from SRX expansions such as SRX 04 (two string sections). (Note: the 1,100 from BK-7m already contain many from SRX 09 World).
All in all, many good sounds are already included in those 1,100 that are identical to the BK-7m. But the BK-7m has some weaknesses in string sections, acoustic guitars etc. For these weak sounds, the BK-9 has got clearly better alternatives among the additional 600 sounds. I would especially mention better strings (from GW and SRX 04), SuperNatural acoustic guitars, better grand pianos (although the BK-7m ones are already really good, one sounding exactly like SRX 11), and a better choice of brass sounds, too.
So, in total, if you pick the best of the 1,700 BK-9 sounds, the sound quality is considerably better than the BK-7m's.