I'm just getting over the flu and haven't eaten much in a couple of days....so this might be completely wrong...
Traditional non-tightbores are 6.08mm...a lot of clones were coming out anywhere from 6.08-6.10mm. I have no idea what a new CA is spec'ing now.
A tightbore (6.03 or 6.04) will increase muzzle FPS...a little (5-15fps)...vs. a non-tightbore (i.e. 6.08mm).
A tightbore will increase your effective range vs. a non-tightbore...but heavier bbs still have a much more dramatic effect.
A longer barrel will increase your effective range vs. a shorter barrel...bore diameter being the same
In some setups...a longer barrel will result in a significant FPS boost vs. a shorter barrel....mechbox & bore diameter being the same. I've setup several M4's this way, where they shoot 385fps with a M4 length barrel and 345fps with a CQB barrel.
So...in your CQB rifle...don't expect to get "long gun" accuracy with such a short barrel....but you do your part, you'll be fine in the field. A tightbore & heavier bbs (i.e. 0.25 min...better yet 0.28) will dramatically improve your effective range.
Cheap and easy upgrade...go for it. Replace the hopup rubber at the same time.
Best of luck...
Tys