ZiPS has Headley at a 268/350/428 slash line. Lawrie at 275/332/461. Not a huge difference. And with all the roiding up the Padres have been doing, maybe Headley's monstrous breakout is sustainable, as long as he's able to maintain his supply. Apparently he pays for better stuff than the aforementioned Everth and Yasmani.
And yes, if he gets out of Petco obviously that's a huge boost for him. But I wouldn't count on that happening, and if it does, it will probably be 2/3 of the way through the season anyway, so you're only looking at maybe a 50/50 chance that 16% of his games are played in a different ballpark which may or may not be hitter friendly.