Well, there's this convo,(http://www.youtube.com/watch?v=UPeI4mX8Nuswhere
) HK-47 tells you exactly how he kills Jedi, and the most effective way to do it. He sounds like he's done it quite a few times.
We also know the relative strength of the HK-50s by looking at HK-51s in SWTOR. It could be argued that HK-47 did take on two sith and two skilled blastermen and (though he lost) put up quite a fight, based on the Flashpoints.
The convo doesn't really say much, again he was programmed to be an assassin jedi killer droid so he would have knowledge of tactics in how to take out a jedi. Saying something, and then executing it are two very different things given all of the variables that can happen between the time the assassin reaches the target to the time the attack is made.
While right, but there is some difference between a 50 and 51. Though your right 47 did do that, based on the Flashpoints(of course we really don't know how it went down since the fight is really gameplay, were probably gonna have to wait.) Am I saying an HK-47 or 50 or 51 couldn't take out a Jedi or Sith?....No, but so far their track record has been less then stellar.