Lately I was thinking about a similar system, using the Join Queue, for the StoreCost calculation. But I added another variable called the Dropout Ratio (the number of nodes that voluntarily* leave the network). Together with the Join Queue could give, in an indirect but simple way, the health of the network to calculate ,in a quite precise way, the optimal StoreCost.
Adding farm bots would only reduce the StoreCost so it does not seem economically beneficial. We would have to consider the possibility of dumping, trying to eliminate competition, although, if the node age determines the profit, it does not seem to be an attack that should concern us.
BTW, in this link we have more recent information about connection speed. Fixed Broadband has increased by 50% during the last year.
*Of course a node can leave the network for a multitude of reasons but I think we can safely assume that it does so because it is no longer cost-effective.