All outside PHB is optinal.
Probaly not, because the sorcerer can burst low level spells slot into Sorcery Point. It's an efficient way to waste resources.
It's literally Spell Point Optional rule with Steroided Subtle Spell.
This is the strongest feature of the game. No competition here.
Stealth metamagicked casting is also viable with Psionic Sorcery. All with one feature.
A level 6 sorcerer has 6 sorcery points. Casting a 3d level psion spell costs 3, applying twinned (or heightened) spell would cost 3 more. The sorcerer is out of points.
As a bonus action, he spends a 3d level slot and gets 3 points back.
Next round, as a bonus action, he spends another 3d level slot and gets 3 more points back. He's at 6 points again, so he can use his cool combo one more time. He's now at 0 points again, and down 2 3d level slots out of 3.
He also has 4 first level slots and 3 second level slots. All together, his slots can be converted (over multiple rounds or out of combat) into 13 points max, enough to pull off his combo 2 more times today.
So, the ability to throw 4 twinned (or heightened) subtle 3d level spells per day by using all of your resources combined (except 1 first level slot, good for a mage armor or a shield). I'm not convinced.
I think you would be much better off not using metamagic entirely. That way you can cast your regular 3 third level slots, plus 5 more third level spells with your 6+9=15 sorcery points. Assuming the spells that can be casted that way are worth it, of course. Are those spells only divinations and enchantments like in the UA?
