I work on 8th near Penn, too! I don't go into Bklyn every day, and if I'm heading that way from my office I usually put up with the utter nonsense on Seventh Ave (not between 5-6 pm though, I'm not a masochist

) or Bway to Fifth at 23rd, then head east once I'm in the village. When I go from the greenway to the Manh. Bridge, I get off at Houston (wrong way on sidewalk just for one block, there's rarely any peds there anyway)-south on whatever that first little street is-to Prince-W Bway-Grand-Allen-Canal. And going west from the bridge to the greenway I usually take Bowery-Broome-W. Bway-Houston. (My avenue route going west/uptown would be to turn from Broome onto Lafayette, then take 14th to 8th.)
In any event, I'm not a speedy rider, so I don't mind convoluted routes if they help me avoid spending too much time on streets like Houston and Canal, which seem to get the worst of both worlds, fast cars and tons of peds, at least at the hours I hit them. From what you've said, you're a much faster rider than I am, so maybe some of that winding around would just be annoying to you. Happy riding whichever paths you take!