Oh yeah dont forget about perform skill. It should have other use than beeing prereq to few songs.

Now it gives dc for fascinate/enthrallment and sonic spellpower.
-No sonic spells worth applying more power to
-dc of fascinate is already so high, that it is not worth to have realy high perform skill

I understand that with sonic spells usability of perform might change, but it should give more.

How about every song(both buffing and not) effectiveness beeing affected by it? And by that I dont mean duration. It would be fantastic if for example inspire courage and spellsong vigor scaled both to bard lvl and his perform skill.