Honestly the thing with kdr's is that you can never judge someone on them. I've beaten lieutenants with kdr's anywhere from 1.00 to 1.90 and have been kicked or repeatedly been accused for hacking. The simple fact is that my kdr doesn't always represent what I'm actually capable of. I'm at 1.17, but the website when I check it at the end of the week usually says that I'm playing at anywhere from 1.30 to 1.50. For instance it says that from May 4th through to the 10th, I played at 1.46, which I think is pretty decent.
If I see some guy who has a 2.75+ kdr, and he's not at least above Sergeant, I wouldn't really consider them an immediate threat.