This is interesting, we also need the small key in that room still don't we? Not sure if the room where you raise the sand is locked without going through the dungeons again myself... But I'm pretty sure we need the key... two boosts seem pretty... time costly if you fail them for a bit and pretty impending on death also if you don't have Blue Mail at that given point in time.
Yeah, we still need the small key there. Getting the boss key early basically allows you to skip entering the room that you use to fill the sand.
However, you can grab that key without moving the gargoyle, making it pretty fast. Just use the bow/hookshot to hit the switch when standing on the red blocks in the bottom left, then throw a bomb to the switch, and move to the blue blocks. You'll get up to the small key, removing the need to go up and move the gargoyle, saving a few seconds.
And the boost really isn't difficult, I can do it without any trouble. (I have an easier time doing it from the corner in front of the chest)
You can also throw a bomb, then use bow/hookshot so that you won't have to wait, the timing is much stricter though.
You could even do another boost to get up to the small key, but that doesn't even seem worth it.
I assume that it is common knowledge by now that you can get pushed up onto any surface with the use of a living armos/gargoyle as long as the Tornado Rod reaches up? (which is why I hate the fact that there isn't one in Skull Woods...)
Edit:
It's not just 3F main room of lorule castle that could to the merge glitch. You can also do it on 2F. So we now know that it works in 3 rooms in the game. all of which has a clear view of a room below it.
I haven't gotten it to work anywhere in Ice Ruins yet however. Which is kind of surprising to me, since it has various rooms that are similar to that of lorule castle and thieves' hideout.
In Ice Ruins I tried to boost from the Northern B2 room into the Miniboss room of B3. However, there seems to be a pretty obvious invisible wall blocking it? I tried with boots cancel and all that, and link "slide" against the air, not moving forward, until he reaches B4, where he regain the ability to move forward and fall down into the void, respawning at B2 again. tried several times, vertically, diagonally, etc.