I did some back of the napkin math for a few coins, and this reduced the cost Ip-90% in some cases. I couldn't find a consistent place to get this data from for all coins, but I may manually go through and add it, since it can reduce the cost substantially as you noted.
To be honest, I think you could even reap the benefits of arbitraging hash difficulty. Eg. just as a new difficulty level is set, flood the network with additional nodes and hash away till the next difficulty update. Then kill the nodes, and wait for difficulty to adjust up and then back down, and then attack again.
