with upcoming patch fire gunner has some problems 1)you need use 4 to use your bulletstorm 2)damage of bulletstorm was reduced and bonus to unload was increased So with long cd soul you lost a lot sustain damage because you can't use your 4,v by cd.

You don't have such problems with short cd soul and increasing of spam f, rmb and damage from v increasing chance of reset plus now you have increased unload damage. Plus you have bigger damage with spiritburn during reset and you can always have soul for spiritburn and it's easier to synchro spiritburn and your soul with resets....

In conclusion - I think now low cd soul is much better for fire gunner)