There's a few different "upgrade" bolts around.
Many people will advise if it isn't broke, don't fix it.
It depends on what you're trying to achieve mostly.
Quieter, smoother, more efficient or a combination. Most aftermarket lightweight bolts will allow you to tweak your settings and operating pressures to achieve varying results.
The bolts I know of are TechT (MRT Gold and Delrin), Boss V2 and the Lurker Eigenbolt.
The TechT and Boss bolts can be used in conjunction with a spring kit, the Lurker bolt is a little more fussy and is best off used with the stock spring.
After adding one of these bolts, you can adjust your operating pressure and dwell to get different efficiency, report and smoothness of shots.