I've seen some people state that Hardline reduces the number of points needed for scorestreaks by 20%. This isn't accurate. Instead, it increases the points you earn by 20%.
As an example, suppose you're going for a Counter UAV. You need 600 Points.
With Hardline, five ordinary kills in Domination will get you the Counter UAV exactly. Each kill nets you 120 points instead of 100. If you run a test, you'll notice that after your fifth kill you will have a Counter UAV and no points built up towards your next streak.
If Hardline worked the way people have suggested, then a Counter UAV would only require 480 points. If that were the case, after your fifth kill you'd have the Counter UAV and 20 points towards your next streak. But this isn't how it works. If you run a private match you'll see that after your fifth kill the little white bar is at zero.
So, the benefit you get from Hardline is going to depend on how you're getting your points. A few examples from Domination: Capping B Flag or Opposing Flag W/out Hardline:
200 Points W/ Hardline: 240 Points Benefit: 40 Points Capture Kill W/out Hardline:
200 Points W/ Hardline: 240 Points Benefit: 40 Points Cap Home Flag W/out Hardline:
50 Points W/ Hardline: 60 Points Benefit: 10 Points Ordinary Kill W/out Hardline:
100 Points W/ Hardline: 120 Points Benefit: 20 Points Destroy UAV W/out Hardline:
75 Points W/ Hardline: 90 Points Benefit: 15 Points
So, if you're just slaying and not playing the objective, Hardline will reduce the number of kills you need to get a Lightning Strike from 8 to 7. But if you're getting your points from capping contested flags, kills while capping flags, shooting down stealth choppers, then you're going to see the biggest benefit from hardline.
TL;DR -- Hardline increases the points you earn by 20% (it doesn't reduce the number of points needed by 20%), so those who take actions that are worth the most points (capping B flag, kills while capping) will reap the greatest benefit.