I have to agree with Styrak. You only really need to replace stuff in mechboxes if they are broken. Most people replace parts because they come with plastic internals. But from what ive seen, they do hold up. CA internals have metal bushings and cylinder head. All you need is a stronger spring and maybe a spring guide. It also takes a while for a motor to die (around 3 seasons of play for me).
As for the hopup unit, im not sure if the CA Augs use the plastic unit the Tm ones do but they can break. If you take your barrel assembly out a lot (which i do for storage), i would suggest getting the GP metal hopup unit. Dont get the PDC one, the airseal on it isn't great. The TM ones have a bad case of misfeeding, but the CA ones are much better. But it still isn't great since its jsut the way the hopup unit is designed.
It is very easy to mess up the hopup unit when installing the barrel. You should take a look for any damage to it. Fully disassemble the unit and see if the hopup rubber is ripped or damage somehow. If it isn't, assemble it again and give it a try. I find if im missfeeding, i can usually fix it by rebuilding the hopup.
As for hopup rubber, use Hard Type.
As for inner barrels, I think Prometheus are the best but they are expensive. However since madbull release the V2 tight bores, there really isn't a need go anywhere else. They are half as much and a 1/3rd the weight. I makes me sad i spend so much on my inner barrels (but madbulls making barrels yet). You can stick with the stock length of 509mm. Its still one of the longest AEG barrels out there. It will improve grouping if you went with a 590mm barrel but it wont help your aim.