Fused Staff states, in part: "You can Cast Spells from the staff as part of a Spellstrike even when the staff is in weapon form. Otherwise, you can't cast the staff's spells while it's in weapon form."
So it would work beautifully with a Starlit Span Magus. I've read that a Staff of Divination is a great choice for Magus due to it having True Strike on it, which significantly increases your chance to land those big spells for big damage.
That does make the Fused Staff feat look pretty appealing then! But for true strike I don't know if that's action efficient, since you can't Spellstrike with that spell. The actions needed for that go like this:
And I'm not sure how you fit a Spellstrike into the same turn without taking an action the previous turn to set up for it. But having a Staff of Evocation or another evocation-based staff sounds like a great idea!
Oh crud, you're correct. True Strike only last until the end of your turn, so you can't set it up for your next turn. My bad on the suggestion, as it works great (and is highly recommended) for a Twisting Tree Magus (which my PC is).