Sorry for the lack of updates, I was busy with some other projects for a while and I haven't had the desire to do any work on this... Well, until now, that is.
Consider this a medium overhaul because I've changed a fair few things.
I had to fix Sonic's head being misaligned on his front-facing sprite when standing still (whoops), edited the "pupil" on his eyes when looking side-ways to make it look more in line with the rest of the angles, added the grey color to a bottom of his shoe in one of the front-facing running sprites while also changing the eyes to not change shapes between frames anymore, overhauled the back of his running sprites by bringing back one of the quills on the back which disappeared every other frame and moved his tail a bit to not dangle too far, slightly moved some of the sprites to be in the center rather than one or two pixels to the side (that previously caused extra problems with sprites that used the mirroring technique), slightly tweaked a falling sprite to not make it seem like Sonic's head is detached from his body, removed some more stray white pixels on sprites where I forgot to get rid of them last time (the only tweak of the super sprites in this update, I'm afraid), overhauled the eyes on the death sprites while adding the missing spines that were supposed to be protruding from the back and many other small tweaks to a bunch of other sprites that would take forever to list.
The other big change I haven't yet mentioned is that I redrew the leg on one of the teetering sprites to no longer look as awkward as it used to be (one of the other sprites in the set still looks quite weird, but I haven't gotten around to it yet). This change is also duplicated onto a similar gasping sprite since both share the same leg and shoe positions. I included an attachment on this post so you could see it for yourself.
However, I've also got some bad news... I had to remove the skidding sprites from this add-on. They are just not of good quality and I've been scheming to remake them for months, so for now, SSNSonic won't be skidding with unique sprites.
I'll try and bring them back for the next update, perhaps as an optional patch that can be applied on top of the base mod. But for now, do please enjoy this update!