ReimuA is without question the best shot type in SA, but there are not really a whole lot of places where gapping is all that useful. Think about it, every attack in the game (outside of the Recollection cards obviously) still needs to be reasonable for all the other shots, none of whom share ReimuA's ability. Even things like the post-pellet hell stream or Stage 6's long pre-Orin stream can be handled with standard crossovers. Sure, gapping helps during those attacks, and a few scoring tricks do call for gaps at certain spots, but overall the difference in how much it helps is not that great IMO.
Here are the places where it saves my ass at 90fps.
1) Trivialize Kisume's spellcard
2) Makes it very easy to collect every power item in stage 1 without doing any grazing or difficult stuff
3) Depends on her movement, but it can save me sometimes on Yamame's first nonspell
4) Makes it easier to get the power items from the enemies right before stage 2 boss without having to graze
5) Rarely needed, but can sometimes help with Parsee's first card
6) Can help with Yugi's blue and yellow nonspells, though that's only if you let them get out of hand in the first place
7) Basically meaningless, but after the bomb which I always use right before stage 3 boss, I can kill an extra orb that survived the bomb with the options that wrap around to the other side
8 - If I'm out of bombs, it be a last ditch attempt to dodge Orin's midboss thing in stage 4 when the spirits explode
9) That hectic streaming part with the wall of lasers right after is easier, and the lasers themselves are easily avoided
10) Can help with pellet hell in stage 5 if I'm out of bombs with nothing left to lose
11) Trivialize the streaming part as well as the kunai crows in stage 6
I never thought much of it before, but playing at 90fps I've come to see it as a godsend.