I have to lean towards some of this. Because I only got singing mastery before lower end All instrument type items went in. If I had happened to get the Djinni Blade before my <Insert large amount of PoA raids here> Then I wouldn't have spent the 18 aa's on singing mastery as it wouldn't have had an effect that would have justified getting it. As with max singing mastery you receive a 1.6 modifier, which is equal to that of the Djinni blade, but that only effects it, if you have no instrument equipped. Now with the melodic gem in which is 1.4 I believe, you would get your best bang for the buck out of instrument mastery as with that you'd receive a 2.0 modifier on singing.
Now to be entirely honest, we have 2 options here.
1) Change singing mastery into something that would provide an effect that would be beneficial to bards. I.E. Spiritplx's idea.
2) Change and overhaul singing mastery and instrument mastery. Make it as they say they are, singing mastery effects your singing mod based on what your singing mod is (even with an instrument equipped that provides singing mod) and make instrument mastery effect every other instrument.
I.E. 1.6 mod from Djinni, with singing and instrument mastery you'd receive a flat 2.2 mod for everything. But if you didn't have singing mastery make it so you'd get just the 1.6 mod from djinni, and the full bonus on all other instruments from instrument mastery.
That way it would give effect to singing mastery, without having any overpower issues. Just separate singing from all the other types of instruments. Although I don't know if that is possible, if not, I would like to see a change to singing mastery to make it a useful AA.