Could you do a similar setup for FPG in the warp zone area in 4-2 to clip into one of the stairs and save time by going straight to the world 8 pipe? Or are the subpixels not lined up in the right way?

Firstly, in the 4-2 warp zone area there is no flagpole to do FPG, you probably meant block clip, and also no the clip won't put you in the block enough to fully clip into it.

I meant do the setup for it, then instead of clipping into the base of the flagpole (because there isn't one), you clip into one of the stairs.

Also, if you enter the world 8 pipe without loading the "Welcome to the warp zone" text, the game will just send you again to the start of the warp zone room

If you want to see example of this go to Happylee's "Maximum coin" TAS. Even though he didn't use block clip to go and went to the world 6 and not world 8 pipe, the principal stays the same. Actually, I had the same idea as you, so I tried to TAS it, the block clip did not work so I tried it with the AISSON Game Genie code, and then I saw what really happens when you attempts to get to the pipe early.

It's not possible to clip into the stairs before the warp pipes. It it was, the TAS would have done that...

If I recall correctly, Mario can't move fast enough to do a block clip into stairs; and in a ROM hack, like Super Fast Bros, you can do stair clips because Mario's maximum speed is capped at 80 instead of 40

Ok.