Reaver
There isn't really a lot that you need to get done at all, you've already got a good hopper & barrel system. I don't really know the air system, so I'll leave that to somebody else to comment upon.
As for mods to your Angel, apart from the purely cosmetic, all I can think of is a Cobra stick trigger. I really like these, they eliminate most of the slack and thus enable you to set the trigger super fine if that's what rocks your boat. Many people buy a volumizer, but it's really a cosmetic mod, although I like them because it saves me 1-2 seconds when stripping down an Angel.
If you're really wanting to mod it, you can send it off to a couple of the custom shops in the States who do a variety of low pressure mods - AKALMP & Bonebrake spring to mind, there are others though - but this involves getting air channels drilled out, etc which might be a bit scary for you. Plus WDP won't fix a fault with your custom parts, only official parts.
EDIT: And you probably would have gotten a better response if you'd posted to the Techroom. All sorts of fruitcakes loiter in Speak Your Brains, it's the only place that doesn't immediately throw us out!