I suspect that it will not have the 50% mortality, and I doubt it does now. Remember, many who get the flu don't go to the doctor they just plow through it, so there are cases that haven't caused death and were not reported.
That being said, for a 50% mortality rate on reported cases suggests a very high mortality on overall cases. The 1918 was under 5%, and caused world wide panic. Once the hospitals get tapped out, if we are looking at something like that it will be devastating.
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.