I agree with you that raiding Arbor is broken. Just tested it – and have to agree that this raid is OP. But seriously – this province yielded benefits three to four times the sizes of gold I gained by raiding bloody Highgarten. And much more than I gained from razing Casterly Rock!
I am quite sure that Arbor should not be more wealthy that those two.
I really, really rarely raid Arbor. Mostly, I enjoy raiding near coastline – and if I feel particularly lucky, I'll go for one of the great cities of Westeros. They all give less gold, they all are much more difficult to raid than Arbor and they certainly result in more losses. (bah! - Sunspear even kills much of your army without battles, by the pure value of attrition)
Anyway, to sum tl;dr : yeah, but that's argument against the Arbor, not against the raiding in general. I just don't think we should modify working systems to match Arbor – if we do nerf it to your advices (to 10%), e.g. pillaging Highgarten (castle) would net you amazing amount of 16g. At the cost of – probably – most of your army, if Reach intervenes. Do you honestly think it'd be worth this achievement?
No, decreasing i to 10% is stupid, you're right. Wasn't my brightest idea. All I think is there needs to be a balancing factor for this. Perhaps a raiding season? I do not know, I am not a modder nor am I creative.