Nothing wrong with the stock bolt unless you are intending to use CO2 and full auto a lot. Then you would want an all aluminum bolt. There used to be a test thread where somebody tested a lot of different bolts for the A5 in a laboratory (university, on weekends, with permission). The differences between the best bolt and the stock bolt for group size/accuracy and air efficiency was so small as to not be worth the money.
Too many of you guys are reading the speedball magazines and think it applies to the A5.
It doesn't in almost all cases. Save your money towards an upgrade that matters - the Palmers Stabilizer. Money well spent, especially if you can only use CO2. It also works on HPA.