Casting channeled acceleration slows your movement speed down to a crawl. Compare this to dark deal, which has an identical tooltip structure but allows you to run at your characters full movement speed while casting. I believe this needs to be reworked because the (extremely short) movement speed buff that channeled acceleration provides is negated by the fact that you have to lose movement speed to cast it, making it, in my opinion, the least useful speed buff in the game.
Here are some possible changes that would make this ability viable for PvP:
1. Increase the duration of the speed buff to 30 seconds, this would put it more in line with speed pots and makes the sustain of this ability more realistic for stamina players. 30 seconds is long enough to let a player fight for a while and then duck into some LoS while they cast the ability, and then go back out and fight again. 9 seconds is not long enough for this.
2. Remove the movement speed slow on cast. Although sustain of this ability will still be difficult for stamina builds, it would give stamina players more of a reason to incorporate magicka regen into their builds, which will result in more build diversity and new styles of play. (This is a good thing)
3. Remove the movement speed buff and rework the skill entirely into a more offensive damage dealing skill. I don't much care for this option, nevertheless, I think it's reasonable for skills that deal damage and offer extremely strong debuffs (dark flare) to require that the caster remain stationary or slowed while casting.