That was luuurverly! I really wish canon had gone like this. Sirius' plotline has always been one of the things I hated most about OoTP. I always felt like he was verging on awesomeness, and then he died stupidly instead.
I love how you've characterised a more 'together' Sirius, who is still a bit of an arse, but has finally had the chance to grow up a bit. And your prickly defensive Snape is perfect. Their developing relationship was glorious.
I adore Sirius dealing with the Horcruxes as your plot flirts with Deathly Hallows while maintaining its independence.