This clip only works in allstars, beacause normally the p-wing doesn't have enough speed to clip, but in allstars you can use L+R to get the required speed. Skipping 7-5 saves about 15 seconds compared to the 100% TAS of the NES version.
...
https://www.youtube.com/watch?v=jBk15WJ-6hk
For some reason this Koopa sometimes inherits some properties of a platform like jumping on him without hitting him or Mario getting pushed by a Koopa like a platform pushes Mario. It seems to be similar to this glitch https://www.youtube.com/watch?v=pyyuCw1dvEc
...
https://www.youtube.com/watch?v=iBLY-igqWfo