Trying for the headshot achievement and finding it extremely difficult for some odd reason. The aim feels squirrely and I can never get more than about 2 in a row before I start missing completely. Once the Boner is powered up more is it easier to pull of headshots?

The upgrades don't make headshots easier, but I just felt as I used the weapon more I got a feel for it. That being said, I never got the headshot achievement. 

Turn the aim sensitivity down. A lot. Aiming became much easier when I did that, though it still wasn't as tight as I'd have liked.

I had my aiming sensitivity at 6 and used the hot boner. The best place to try that achievement is in Act 3-3 I believe. The one called "My Dying Concubine." I think it's right after the part when you discover that darkness kills the giant cyclops demon. After that you should be walking across a walkway over the water that collapses. Then you see Paula drowning. Don't chase after her. Enemies will keep spawning in the water as long as you don't trigger the strawberry to appear. So if you trigger the cutscene where Paula gets dragged across to the other side of the pond, it doesn't matter as long as the strawberry does not spawn by causing her to disappear. So you have infinite enemies spawning out of the water 3 at a time. Just keep trying until you get it. It's not that hard if you keep rolling and dodging until you are certain you get a clear shot where the enemy isn't moving as much. It'll take you a couple minutes, but hey, you'll farm a decent amount of diamonds in the process.

Turning the sensitivity down helped me a lot as well.

I did it in act 1. There's pretty much no consequences there and you can take as much time as you want to pick your shots.