Jammy
If you're new to the IR3, I'd say just leave it alone! Play with it as it is, you'll find that the basic marker is excellent, hence the reason why there are very few worthwhile, non-cosmetic add-ons.
The standard barrel is already aftersales quality and so the only realistic reasons for replacing it are either cosmetic, poor paint match with the paint you commonly use or a longer/shorter barrel to fit your playing style. You aren't otherwise going to get a real performance boost simply by changing to another 14" .687 barrel.
The trigger on the IR3 is very good already, against that the Cobra trigger for the LCD was a big improvement over the stock trigger, so it might be a worthwhile buy if you are uncomfortable with your current IR3 trigger.
As far as volumisers go, a very common addon on Angels, they really are only a cosmetic change apart from one minor useful feature if you do your own tech work: they're a lot quicker to remove than the standard end plug & don't require an allen key to do so (a minor point, but I'm very lazy
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Some of the cosmetic sight rails, back plates, etc that you can get look very nice - check out some of WDPs own addons.
If you've got money to spend then I'd look elsewhere, perhaps a new air system or hopper upgrade?