Tell me something I don't already know... This has been said by her producers a thousand times already (even those who were cut from the albums and in spite of being pissed off against her team). Britney always had a wonderful tone that suited all kinds of music perfectly. She's more of a vocal actress than a "singer" which is way more difficult that it seems. Of course, in this bombastic world where the louder the better, only a few really appreciate Britney's talent.
If Sia's song showcases Britney's "singer" side, I hope it'll be as wonderful as that: