I really should have posted this years ago, but I have a short WIP that I abandoned some time ago that is 5 seconds faster up to the entrance of MK castle during Mack. It doesn't have the Mack skip though.
http://dehacked.2y.net/microstorage.php/info/1689367062/smrpg.smv
The main differences are:
1. The first battle: move the cursor to a different Terrapin and you can kill them all without getting hit once.
2. MK Castle: don't bother getting the frog coin. I didn't get it because in this WIP I wasn't planning on getting the Exp.Booster, and the Crystalline and Power Blast can be bought using the frog coins found at Midas River without losing time. It might be a better idea to get the frog coins needed though, because closing the Exp gap with the Lucky Jewel/Lucky Hammer (with the "find yoshi" crap at the end of a lot of battles) might end up taking more time than needed. Or, it might be faster to not get it because being 2 or 3 levels lower might not cost so much time in the long run seeing as how once you get the Rock Candy its one of the most often used items thereon (first place it's used is Knife Guy Grate Guy battle).
3. Entering MK after Bandit's Way by moving to MK, then moving back to Mushroom Way, then back to MK then entering the level. This puts Mario at the western entrance closer to the entrance of the castle.
There might be a few more time savers in there but that's all I know of.