How are you defining "reasonable amount"? By my calculations, StF needs something over 71% Mastery for its direct damage to beat the DPS of an empowered Starfire in CA.It will depend on tuning, but if the relative initial damage of Stellar Flare and Starfire stays the same, Flare will be much stronger with any reasonable amount of Mastery. Especially since Blizzard intends to tune Mastery as our strongest secondary.
Stellar Flare Coef: 94.5%, gets to double-dips Mastery (*1.71) and has a 1.5s cast, giving 107.7% SP/s, before counting bonuses that also apply to SF.
SF coef: 186.75%: 20% bonus from perk, 30% bonus from empowerment. Currently a 2.7s cast, comes in at 107.9% SP/s.
At that point the SS you used to empower SF comes in at 280%/1.5s = 186.7% SP/s. You've also got three charges at the start of the fight. No reason not to use one early in CA.
If haste is a better stat than Mastery, and you manage to have 71% Mastery, you may also be nearing the point where flare is below the 1s gcd cap, and that would also favor SF.