Just because they come with a 6.03 tight bore doesn't mean its any good. For one thing its brass and not steel, and for the other the barrel itself might not be all that straight or finished very well. I haven't yet proven my barrel is at fault for my accuracy issues, but with FPS consistent (within 3 fps) and a g hop in there I am now leaning towards my wonderful 6.03 tightbore barrel from G&G being the issue causing terrible accuracy.
Tightbore != accuracy
Good quality = accuracy whether its tight or not
What a tightbore does is use gas more efficiently, so it raises FPS. We have good video evidence that the BB travels along the top of the barrel regardless of the inner diameter, and that minor issues in that can impact accuracy and range.
So I wouldn't say there is no upgrade available not at all. But at the very least test your barrel and determine how the accuracy holds up compared to other guns at range before you determine there is an issue.