To be honest, personally I would be more inclined to add aspects and elements from Deathwatch into Only War than vice versa, but this is largely the result of a bias originating in my appreciation of OW as the best of FFG's 40k RPGs yet, as far as rules are concerned.
Either way, perhaps OW's approach in regards to Full Auto modifiers would allow to bring back this firing mode for the boltgun?
As for the heavy bolter and bogi_khaosa's quite understandable concern, one way to address this would be to simply rewrite the Bulging Biceps talent in that it allows "bracing without support" at all, rather than this already being included in the Bracing Action where - for some reason - the rules do not make any difference between a person just standing in the open or actually supporting their heavy weapon on a bipod/tripod or at least a piece of the environment.
This would then replace the negation of the -30 penalty for firing an unbraced Heavy Weapon, requiring the operator to combine his Half Action Autofire Burst with a Half Action Brace, prohibiting additional movement.
As for the side-issue of the "realism" in autofire granting a greater chance to hit, I agree with Wonder Lemming. Obviously, the accuracy of the single round does not increase, but with an entire salvo being fired it becomes at least sensible that the ruleset would make it more likely that some of them hit their target by sheer coincidence. How much greater this chance should be is, of course, debatable, but I don't see how the basic principle should be wrong.
And this is before we even delve into the question of whether these sci-fi guns have notable recoil at all. The various sources of fluff are quite likely contradicting themselves on this detail, too, but considering what tricks are available to compensate for recoil today already, it is an option to consider.