I'm going to quibble with the question. The new standards in monster design are not errata. Whether or not you decide to beef up monster damage is definitely your decision, (and it's perfectly legitimate to want to do that) but the harpy stats aren't "wrong".
To use the exact examples you've used here:
Here's a Harpy Screecher.
Here's an MM3 level 9 controller.
From what you can see, the stats (hp, defenses, to-hit..) are mostly comparable, except that basic attack- which is about 8 (average) from the Harpy and 15 (average) from the Stalker. I think the Stalker has a more powerful encounter power (the Horrid Link ability, vice Acerak's Slave), but otherwise the Harpy's screechy pull and slide powers and the Meenlocks dazing and quasi-dominating powers are comparable.
If you really wanted to beef up the Screecher I might add some damage to her basic attack, but it might be even better to give her something like a minor action rechargeable encounter power- perhaps something that chains off of slide or daze, minor action, rechargeable on a 4-6. It doesn't even have to do damage. A fortitude attack that gives a character Vulnerability to Thunder damage or reduced defense against charm attacks.. would be great.