Don't think the verse or most in it should be ftl or ftl+. At best Light speed and that's Ragna with his greatest attack.
So for starters, Ragna is requiring from himself an attack that can be released in a single moment and decides on
Light Speed
0.000.0001 seconds is sub relativistic perception time, something Kamui couldn't react to.
Next the very obvious; it's stated his greatest attack was the speed of light.
After this Kamui is then astonished and outright states it's something he couldn't possibly imitate. Further on then says "I can't even possibly imagine how you brought this move into realization." It's pretty clear what the author wants here, it's pretty clear where the characters think they are in terms of speed themselves.
I think where Kamui and Ragna can scale is obviously mhs to mhs+ just going off of Kamui's base speed being mhs+ with him doing this
Mach 29,154.51895 or high into Sub Rel
The peak of sub rel being Mach 43,701.52
And of sub rel+ just gets even higher from there.
To show a difference in speed from where Kamui is at,
Speed of light Mach is 874,030
While Sub Rel+ Mach is 43,701.52
Being a 19x difference in speed.
Another thing to add is that Gilzea also says Ragna's striker could kill her