The rubber sleeve IS the bucking, what you might be thinking of is the nub (the tiny little rubber tube that applies the pressure). The stock nub should be fine unless you're running a flat hop/R-hop setup, in which case you'd need a flat type nub, the same type that's helpfully packaged with the Modify set you've linked to. I have no personal experience with flat hop setups, and R-hop is more of a DIY thing that takes a lot of effort, but yields great results, I encourage you to look up R-hop but for now it's probably not worth the effort.
As for the nozzle, a properly fitted air nozzle with an internal rubber O-ring is used to improve air seal, which improves shot-to-shot power consistency which potentially improves accuracy. Also, if your current nozzle misfeeds a lot, a new one can potentially fix the problem, but if not then it's more likely to cause feed problems than to fix them. On it's own, a new nozzle won't give much noticeable effect, it's only when all the other pneumatic components have had their air leaks fixed that you may notice an improvement in consistency. Air seal is more a problem for marksmen's rifles, if you're interested in the topic read more
here but be warned that pursuit of a 'perfect' air seal will consume many hours and cause lots of headaches. At least teflon tape is cheap.
Something that might not constitute an 'upgrade' per se but which will probably help out quite a bit: get a cleaning rod (I got mine from Canadian tire) and clean out the gunk from the inside of your barrel. A new barrel comes packed with grease from the factory, you've had yours for a while so probably all that's left is just a thin coating that would get pretty dusty. Clean the barrel before and after each game, keep it clean, dry and free of scratches and you'll find it works a lot better.