I think there are 2 problems:
1) Timout. After about 60 secs or so it wont go on. With a slug its not easy to deliver the entries fast enough. Latest nightly plus sqlite3 selfcompiled is likely best. 4.3k work in about 15-20 sec so at around 12-15k it should time out.
2) Memory in the Soundbridge. They never stated a maximum precisely but the thing has only 32MB memory of which only so many is free. I think over 10k you get a problem with that anyway.
So basically, much more than 10k songs in one select wont work. You need to either split databases or only use playlists/selections of <10k.