We're seeing spikes to 1MB now, even without driving events we can point at. So, at the moment when 1MB definitively isn't enough, I fear that 5MB won't be enough either. Transaction volume in the real world is incredibly spiky in response to such events, and Bitcoin is starting to have closer and closer interactions with the 'real world.'
So in your formulation, a single block is a meaningful spike?
If so, there isn't a max block size that can accommodate.