10%? Seriously..., you're joking, right? Every other Lord Paramount can have 10% of the income of Iron Isles. And, as far as I am aware, none of them is reliant on the raiding for most of their income.
But then, in last 20 years in Aegon's Conquest scenario I had two bloody wars, every one lasting as short as few months. And I believe that most of the time I had depleted levies, because of raid on Highgarten going awry – which is double painful, as Iron Island are quite possibly the smallest Lord Paramouncy.
I'd agree with some nerfing of raiding, but I think it should be not more than to 40-50% of base value. Raiding should make it possible for Ironborns to accumulate more wealth than their neightbours – otherwise, it'd be just irritating. Hey!, it's not like you can raid anytime, don't risk your armies on raiding and (with new levy reinforcement rate) won't be prevented from significant raiding for a few years if you won't pull off from North before this crazy-sized Northern army wrecks your.
PS. Also, I dislike rejecting interesting and flavorfull features just for the sake of 'this group is OP'.