You're missing the point.

They're rated by windspeed because of the damage that certain speed winds can cause.

http://www.aoml.noaa.gov/hrd/tcfaq/D1.html

Accordingly, anything beyond 155 will continue to cause catastrophic damage. There's no need for a "super 5" because a 156 mph storm and a 220 mph storm will each cause catastrophic damage.

Until there are new advancements in building design and structural integrity, there's not going to be a need for a 6.

When we get to the point when a "cheap" house survives 200 mph winds and 20+ foot storm surge, drop a line to the NHC - I'm sure they'll consider it then