It seems that the main advantage of this is that you have one thread manging many sockets. I am a bit surprised that blocking kthreads would be so much slower relatively. What causes the slowness? Context switching? Additional stack memory usage?
With an event driven architecture you only have to hold the session information per connection in memory which can be as low as 4K, therefore you are able to maintain (depending on the complexity of the protocoll) n*100K connections.
The only drawback that you have to write and think your whole program event driven; you write callbacks for each io and timer operation and the control flow won't be clear if you read the program.
Anyways, I was talking about C, where the stack is allocated when the thread is created and you cannot resize it later, nor it will grow automatically.
With dynamic languages it is different, but don't expect handling that much connections with them either.
Typically an output buffer should be able to hold the complete production for a client, if you limit it to say 4K you will be unable to process a request in one go.