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...