Do you see a practical reason for me to have a SG220 and a SG160? My theory is to skip the mid
I'd say definitely get the 280 if you have the pk40. You'll be able to do most trees with it performing fewer cuts. The one drawback of having just the 280 is the length of the bar when removing bushy trees. You have to be very careful not to cut nearby limbs that aren't in your grasp.
The 220 is a good all-rounder. Big enough to do most trees. Not so big that you have much trouble with bushy trees.
Keep in mind how they work. The weight of the saw dictates the weight of the piece you can cut.
The 160 will be great for long reaches and bushy trees, but will be limited by weight. You'll probably want to stick with limbs weighing around 400lbs to be on the safe side.
So you see, using the 280 on a nice open structured tree can probably cut your time in half compared to the 160. You'll be able to grab and cut much bigger pieces.
All 3 of these heads are bigger than the Altec grapplesaw, by the way.
I'm looking forward to getting familiar with the sg160. I'll be making posts on Instagram using it on my excavator pretty soon.
Having 2 is a good idea. I may even get all 3.