The only metamagic you should apply to PW: Pain are the ones like Fell Animate where you get a bonus effect aside from damage, and even then maybe not so much. The idea of scribbling up a bunch of scrolls (or even a Wand) of PW: Pain is a good one, since personally I don't think it's worth a spell slot most days.
By the time you get 2nd-level spells, it's pretty worthless. It'll take it's full duration to kill anything with 30-50 HP, and above that it's not effective at all. That said, if you have a fleeing, wounded enemy, PW: Pain is a good spell to throw on them before they get away (if you can, it's a close range spell).
The exception are Warmages. Even if the effect is minimal, Warmages can do tons of damage with this spell since Warmage Edge applies on each round. At higher levels, you can cast a spell with big up-front damage like an Orb and then follow it up with a quickened PW: Pain for a no-attack, no-save method of finishing them off.