I agree with this 100% and then another 100% on top of that, if I ever went from ViP to Prieum, the first thing I would buy would be my Shared Bank again, if that meant I had to do without content for a while longer, then so be it.
It is really one of the best things for a player can own.
Which is why I disagree with your idea, I do not, or could I ever, support a one time payment of as little as (currently) 7.99 would entitle you a permanent 5 slot shared bank account on top of getting the TP from the purchase. That would be WAY too much to a premium member and take away too much from the benefits of being a ViP.
However, I so support the idea of 200TP Single Slot shared bank account.
I think if they did this, they should make it open (IE: you can use it like a normal shared bank slot) or if they put a timer on it, make 2 day 18 hours, it be inline with a raid timers
I very much would get behind this idea. I would never buy it, ever, as I know without question, that I would buy the 20 slot shared bank, but, I think it would be something that players might like, a little something to let them trade those BtA items. Without big scary 1K+ TP for the 20 Slot.