It's not that simple. Buying 8 servers with 1 gigabit ports doesn't mean all 1 gigabit will be used. In fact, from discussions I've seen, that's guaranteed not to be the case. But my objection has more to do with the fact that adding 50% bandwidth to the network in a week, or even a month, would be noticeable. An attacker would have to spread it out over several months, greatly increasing the cost of the attack. And you have to factor in the 12-18 servers needed for HSDirs, and the computational cost of brute forcing their fingerprints to be closest to the descriptor ID.