First: I really like this setup and I think it does suit everyone's needs, and the ability to move the perm slots around (even if it does accrue a moderate charge) to me shows that you're doing this for the users and not just the cash.

My only disappointment is that this still leaves nothing between the free account's 15 icons and the paid's 100. That's a huge gap. I read all the linked articles about why it's not cost effective and too much of a headache to offer extra icons on free accounts so I'm cool with that part, but would it be worth considering bumping the amount of icon spaces on a free account at some point?