Granted, the stipend and the slight game improvement provided have yet to make the gold account feel like appealing choices to pay for versus the silver accounts and whatever those people could earn via microtransactions.
y'know i hate to be the one to break this to you guys but when a game goes f2p the most important people aren't the subscribers. indeed if you go f2p in pursuit of subscriptions (which is what many subscribers and bad mmo developers tend to think) you've basically already lost the point of what you are doing.
what's important in a f2p game is not pandering to subscribers but rather providing the most attractive experience to the vast unwashed hordes of f2pers who will never subscribe to your game but who at the same time will provide the vast majority of your future revenue.
that laid out there although this doesn't seem like a big deal and is compared directly to bag space etc the fact is doff space directly relates to the amount of doff missions you can do and thus progression speed through the game and thusly should be more carefully considered than striking off a dozen bag and bank spaces here and there.
I love this idea and it doesn't have to be a huge boost each tier, just a small addition of maybe 10 per tier would be nice and may even encourage purchase of more space if a value is noticed from the free addition.