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Why on earth would you need to take the transformation that is faster than the transformation that is faster than the speed anyone else can have AND MAKE IT FASTER...
Because Shinobi Legends (which all of us pretty much are) do that kind of stuff regularly? Orochimaru, Kabuto, freaking Madara and Obito, stacking stuff on top of other stuff is by no means nonsensical when the average, powerful fighting character is on par with the aforementioned, if not superior.
Why do Rinnegan users need to have any buffs period other than the 6 paths powers? Those powers alone wrecked Konoha in the canon. Why combined senjutsu and tailed beast chakra? I could go on a few more examples, but I think you get the jist.
I've had people ask me, "How would you beat someone using Raiton no Yoroi, Sage Mode, and 8 Gates for super speed?" and I think to myself man that is dumb. I do not think we should continue acting like all of these speed buffs stack.
Sage mode and nine-tails cloak stack in the canon, so I don't see why those wouldn't stack. It might kill the average person, but if someone is claiming all three of those it's fairly unlikely that he/she is RPing as an "average" person.
...Lightning Armor should be the fastest speed to anyone not the 9 tails host (even then I rp'd it as just a notable step up from Raiton no Yoroi, not that it makes me The Flash)...
If we're going to be technical, Madara and Obito had little trouble taking on the speed of Naruto's 9-tails cloak. Be it a silent nerf or whatever, the only thing, speed-wise, that caught them off guard was the likes of hiraishin (Tobirama and Minato). And many Uchiha-Rinnegan users RP at the level of Madara and Obito if not greater and have the sensory skills (extrasensory) to sense it coming. So it's no wonder that in order to try to use speed as a viable tool you have to really, really, get ridculous with the speed buffs in order to have an argument that "bruh, I'm faster than you can react" with any top-tier Shinobilegend.
If RNY is going to be used as the basis as the fastest we can go, EXCLUDING things like KCM, and other allowed cloaks. How are people without KCM or other not being named cloaks, counter those people who are explicitly faster?
Twitch jutsu. Maybe you're faster than I can move, but not faster than I can think and expel chakra. Hiraishin ftw.