The potential for disparity is ability terms is the utility in celerity which is incredibly good and the cast time in soultfire which is incredibly bad but cant be properly fixed but as a band aid WP have accelerated intimation after using Hammer of Sigmar which cuts cast times by 50% which can serve to facilitate soulfire (and other things such as a fast rez).
The thing is, that soulfire is a very weak dot (I have 20 points spent in wrath tree and it's 738 dmg over 9 sec), for which to apply you have to stand in the middle of the fight, so if setbacks are taken in the account it simply isn't worth the hassle. What's more - there is no synergy between Soulfire and Hammer of Sigmar as HoS should be used after Soulfire is applied so it could hit with the dot. The lack of synergy between abilities is one of the biggest problems of melee WP + most abilities require a tactic to actually work + crit values are low. I don't think I ever seen 1k crit on anything other than undergeared SH or Sorc with my WP, even when I ran around 800 str unbuffed. By comparision DoK critted me today for like 1,2k with Wrecking Agony (pre-mitigation value) in 1v1 situation. Maybe this is worth looking into?
No Wrath WP can justify dropping DF, Fanaticism, HD or IR for guilty soul. It's not a tactic that is usable because there's no room for it.
That. Plus dot values are too low while the tactic procs out of only few abilities. If it procced out of AA it would be little bit more viable option although still dropping IR would be painful.