You can't spend 400 BTC 80 times in 1 hour. If you control a majority of the generation you could spend them twice an an hour (assuming merchants require 6 confirmations).
Why ?
There are only, on average, about 1 transaction every 10 minutes. That is six confirmations in an hour which is the hard time limit before a transaction is confirmed. Let's assume the attackers are real lucky and get twice that many blocks as they are doubling the generation rate.... hence only two attacks would be possible in an hour. Maybe a few more than that by "double spending" on each block, but you would also have to "win" all of those blocks too.
BTW, attempts to double spend coins would be ignored once the "attack" is over with likely a bunch of competing chains floating around the network temporarily while nodes are analyzing the transactions, but once the double spending filters are applied to the transactions the double spending transactions and blocks associated with those transactions will be culled... as if the attack never happened in the first place.
It would essentially be wasted CPU effort and even the bitcoins "earned" by winning a block would be discarded too. It is as pointless of an attack as I've ever seen proposed.