The present, unmutated virus kills at 50%. If you get it from a bird, you've got a 50/50 chance. Then again, you are right, that's reported cases -- if it is severe enough to go to the doctor, 50% of the time you are going to die.
Now, again, you are right: If it mutates, the lethality might go up or down. We might end up with a 90% killer bug, or another 1918 5% killer bug. A 90%-er would have us wearing bearskins and carrying clubs in 3 generations, and a 5%-er would really collapse a lot of economy.
On the NOVA special last night, they interviewed people from rural villages, who did seem to have a solid lack of awareness of the seriousness of this flu, yet who all were aware of it. It leads me to believe that in Southeast Asia, people all know about this deadly flu and that they WOULD seek medical help ASAP. So I do not think that "many cases" are going unreported.