<<This may be a basic common sense question, but I wanted to get confirmation:
<<Lets say a PC has all the magic mastery feats and is capping out on spells. Thus, exp is not as much.
<<Can said PC forget those feats to lower their max cap, thus learning more.

No. The feats don't extend you past the spell's hard cap, they just increase your personal cap up to it. I.e. they're effective skill boost to your Augmentation, Utility, etc. skills. That said, if you're capping everything then there's no point to knowing the feats anymore, since your caps will be the same with or without them unless you're right near the breakpoint. One thing to bear in mind is that the feats do affect sorcery and symbiotic personal caps too, so unless you're capping absolutely everything then they'll still have an effect. It's just a reduced one once you start to be able to cap normal casts.

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<<Do we know how much of a boost the mastery feats boost their respective magic skill? Is it percentage based or flat rate boost.

Honestly, I have no idea. I would assume percentage based, since certain guilds get them for free at second circle, and it has to scale across all skill levels.

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