World 1-2: just walk right on through this pipe to the warp zone, nothing weird about that!
World 4-2: take a stroll through blockville. Just like pipeville!
World 8-2: getting hit by bullet bill next to the flagpole counts as finishing the level?
World 8-4: It looks like the piranha plant is still visible in the pipe when you go down it (4:20 game clock) :O
As for ways to shave off more time, there was one level (8-1) where you hit the top of the flagpole. Could it be faster to fall to the bottom before touching it?
Great stuff!