In short depending on the streak it will take 1-2 less kills, one less objective capture.
If you are playing objective games, the extra points for getting defends, captures, capture kills etc will be amplified.
It takes about 16.67% less points to earn a streak, and does not work with assist points from things like UAV and CUAV's.
Hope that helps, personally I find it quite noticeable sometimes, depending on the game a bit... But this might depend on the game type you play and how hard you play the objective.
PS I tried to post a link to a YouTube video that explains it, but it wouldn't let me. If you go to Drift0r's YouTube channel, he has a Black Ops 2 "In Depth" playlist, you'll find hard line and many other explanations in there (I think it is within the community rules to post stuff like that if it is on topic? If not let me know and I will remove this last paragraph)
Its pretty significant.
If you run hardline and get a 10 gun kill streak you will have a VSAT, as opposed to needing 12. A swarm takes 16 (or 15 and an assist of some sort) as opposed to 19.
Its pretty noticable when you don't have it on. A few times I put on flack jacket on instead and get see the merciless medal and think - 'hey, where is my VSAT??'.
Its not very usefull for lower streaks at the front end- as it is based on percentages not kills (like MW3, it took one kill away from whatever streak you were going after). It pays off huge in the back end though.