So should I be looking at dropping from a 170 to a 167.5 main?
I'm using the 167.5 right now, and the clip is in the fourth. I'm thinking about going back to the 170 in the future, and moving the clip to the 3rd, with an extra washer under the plastic spacer for the mid point between the 3rd and 4th grooves.
Erbman has posted before, that if you stay out of the throttle, the larger mains aren't that much of a problem, but if you want the performance it'll be there. I can't tell if there is really a difference performance wise between the 170, and the 167.5. It may be slightly less impressive with the 167.5.
You might try adjusting the accelerator pump per this article.
This made a huge difference for me. The blue(duration) screw in the diagram, on my bike is 1 1/2 turns off of the locknut now, with the nut on top of the stop. I tried the nut under, and the screw on the stop, but it was sluggish. Its pretty decent right now. A good compromise. It also helped with the torque snap in first gear. Its alot smoother. Everytime the throttle is twitched, the pump spits fuel, and alot of it, at the factory setting.
As has been said, 70mph is the cutoff for economy. Above that, and the fuel is flowing.