Uh...this would be overpowering. Let's say the SMAW has eight shots again. All eight kill at least one person, that's eight kills. Kill two more bad people with your secondary and you have another 8 SMAW bullets.
they should just add bonus xp at points in your killstreak, so the bonuses would go like this: 1-100% 2-100% 3-150%+3 bonus xp on the spot 4-200% 5-250%+10 bonus xp on the spot 6-300% 7-350% 8-400% 9-450% 10-500%+20 bonus xp on the spot up to 15-750%+50 bonus xp and 20-1000% +100 bonus xp reaching a 20 kill streak is worth 394 xp, which is hard to do, and is a decent reward.
I think they need to fix their xp system. Personally, I don't feel its very rewarding to get only 2 points per kill with only a possible 10x multiplier to that. If they make us have to get over 50,000 points again to try to get to the highest rank then I will despise the system.