Let's be honest with ourselves, I don't think insta-conversion from a combo to a kill would be very balanced on Marth, especially given how early Marth kills. I wouldn't necessarily say Sheik's triple/quadra f-air combo is a really FAIR (oh god that was terrible) combo seeing as how it just dominates stage control, or Diddy's incredibly annoying strings and early combos which leads to Diddy just making it incredibly difficult for most of the cast to handle him.(including not incredibly good Marth's)
While I'm not saying removing some lag on his aerials to provide him with a potential double tipped f-air combo of his own would be OP by any stretch. I am content for now with that sourspot n-air to tipped f-smash, which hits most heavy characters (at least Bowser, Donkey Kong, and Samus) at around 40% and ends up dealing a ridiculous amount (I think around 80%) and still provides quite a good knockback meaning you can zone said heavy character away from the stage. However, even with lighter weight characters like Lucina and Peach, you can still strong some f-airs, and use a nice f-smash RIGHT when they are going to grab the ledge for a KO at even 53% which is just ridiculous. (If I can get my laptop to work I'll probably end up showing it)
Something to test out, but I'm trying to see if you can reverse up-B, if so, d-throw to tipped Dolphin Slash will be a viable true combo starting at the 26% to anybody not Jigglypuff basically and has enough knockback at that point to become not easily punishable. Time to hit the labs I suppose.
EDIT: NEVERMIND CONFIRMED YOU CAN. :D