I wouldn't say this is a TF2 issue. To me it sounds like a streaming with one PC issue. Sure, an i7 will stream 720/60 "no problem" but what people fail to mention is that you'll still have this slight mouse input lag. Go ask any streamer on one PC and they'll admit that they have this issue. The only way to solve it right now is to get some kind of super computer or stream with two PCs. Most people who stream with one PC just put up with it because it's not THAT bad and it's worth it to stream with good quality. I did it myself for two years before buying the second computer.
I'm pretty sure this input lag is one of the reasons why a lot of the top TF2 players don't stream their big matches. They're willing to stream matches against easier opponents because the input lag won't make much difference on the outcome of the match.