Before I go into specifics, its not unblievable Itto Rasetsu did make him FTL.
Lets go into specifics of the anime, Ikki at base could react to supersonic + speeds with his eyes closed without the use of Insight, Perfect Vision or Played Steal.
At Itto Shura, that would make him hypersonic, which would make sense since Itto Rasetsu would make him massively hypersonic +, which it did, him being able to outpace the speed of lightning.
HOWEVER THERE IS CONTEXT. Ikki's Itto moves requires the release of ALL his magical power. People forget that he has spent a week+ fighting everyday without sleep or food. His stamina was extremely low, and until he had support from friends he was hallucinating. The dude had NOTHING going for him in that fight, and his magical power was severely drained. As we know from Phantom Swords, one can drain ones physical energy by striking and draining their magical energy. Both are interlinked, so with this I can safely say his magical reserves were heavily depleted (even without this fact it would make sense, he had no food or rest for a week+ and was fighting every day, exhausting his already little magical power). Which means he outpaced lightning with a very weak Itto Rasetsu. Keep this in mind as I go forward.
Then came Edelweiss, Edel was FTE to Ikki when he was in Itto Shura. But after copying her style, he was able to keep up with her when he was still in a near dead state of body and mind. The dude was beaten to a pulp, and was still achieving massively hypersonic speeds (kept up with Edelweiss without Itto Shura) meaning imagine the speeds hed hit at base when healthy.
This would mean Itto Shura, after copying Edelweiss style (which allowed him stupid more amounts of body control) hed be at least massively hypersonic + (this is from the numbers hed mentioned that he is boosted by) so its not ludicrous to say the 100x boost from Itto Rasetsu would make him FTL (if lightning is 1/3 speed of lightning).
NOT ONLY THIS shaky powerscaling as it could be called, Ikki became a Desperado mid fight against Stella, which gave him stupid more amounts of magic to put into Itto Rasetsu and subsequently Oikage.
What I mean from this is that, on top of that one feat, people are saying one feat (calling it shaky) is not enough to boost him to FTL. However, we have powerscaling backing up the fact hed be FTL when he uses Oikage and Post Desperado Itto Rasetsu.