Sounds like you've nailed the problem, though I'm no tech. Some here will probably say that your switch to synthetic probably exacerbated the problem, as switching on a high(er)-mileage car usually does just that. At the amount you're burning and with no leaks, I'd say it's time to pull the head. As to cost, I don't know (yet) - but I'd imagine $1400 - $2000 if you're not a DIY'er.

Correct me if I'm wrong, guys...