Two of the KWA's that I've come across needed a ship on the sector gear, and almost every KWA (7-8 to date) that I've worked on had lateral play in the bevel gear. Was it going to lead to a catastrophic failure...can't say 100% for certain. Was it nice to know that things were setup nicely...100% for certain.
For the ones that needed shimming...it may have been from how far the bearings were pressed into the mechbox shells...it may have been from variances in the gears or the bearings themselves. From my determination, they needed reshimming (and I do know from feedback from some of the owners that their rifles are still going strong).
But, most certainly, they were all quite nicely setup right out of the box (i.e. worlds ahead of junkers like Dboys...or even nice setups with nice components like G&P complete boxes).
You can take the most precisely machined piece of anything and put it into an assembly, where upon it's precision and the ultimate end result is immediately dependant on much more than any one components precision. Factors such as QC and tolerance stacking come into play.
Certainly good enough...is good enough. But there's always better...
My statement to the effect that checking the shimming of any AEG (new or used) is a good idea stands (but you are certainly free and most welcome to disagree
!).