Concerning the sewers OoB: I just noticed that it might me a bit faster and safer to use this valve-thingy to boost onto the pipes and the connection between the two pipe-parts to go OoB instead of using the chair and/or the piece connecting the pipe to the wall.
the reason i personally use the chair isn't to get on top of the pipe. if you have the back of the wall on the pipe side you can boost up the back of the chair, get an extra boost off of the pipe where it curves into the wall, and then fly out of bounds. at least in my experience it's been way safer.