Even though each cluster was single AZ the whole system wasn't, so we weren't bound by the capacity of a single AZ.

Most of the situations where we needed to drastically scale up were known ahead of time as well (e.g. campaign from customer), and we would preallocate instances or even more clusters.

I may be forcing my memory, but if I'm not mistaken, our auto scaling was setup in a way that the system could handle sudden load increases of ~50% without noticeable disruption. Spikes bigger than this could lead to increased latency and/or error rate.