Actually Bleach does make prolific use of the super blurry after image lines and supar teleportation speed. Aka Shunpo.
In terms of speed, by comparison, Cloud dodged a fireball from Bahamut SIN that traveled the length of a city in a moment (I've seen claims that the fireball traveled at mach 26 or some shit), and Sephiroth is faster than him. He is definitely much faster than Zack, who can deflect hails of bullets coming at him from multiple angles.
In terms of raw physical power I would agree Kenpachi has the edge, but Sephiroth's feats of cutting through dense layers of metal and skyscrapers shows he can compete in terms of striking ability.
More than that though, Sephiroth can fly, which Kenpachi has never shown the ability to do (Other Shinigami have, but Kenny has basically little control over his Spiritual Power, we can not say that he could because they could), has ranged attacks (Sword beams, in Crisis Core materia, in Advent Children the Lifestream itself as well as telekinesis), really, Sephiroth could use his telekinesis to hold Kenpachi, and even a moment's respite would allow Sephiroth to make a killstroke, and even if that fails will be outside of Kenpachi's range to harm. Due to Sephiroth's longer sword of course.