With the specialist strike package definitely use attachments because after 7-8 kills you get the kick proficiency along with all other proficiencies and perks.
With support or assault strike packages it's up in the air, but this video says grip is better:
I do exclusively use the Specialist Strike Package (on every class I've created). However, from everything I have read online, after 8 kills (7 with Hardline as you stated) you only get all the Perks activated... not the weapon Proficiencies.
Can anyone confirm one way or the other if you get or don't get all the weapon proficiencies? Everytime I have gotten the All Perks Bonus I have not seen any difference in the proficiencies other than the one I am already using.
Looks like there are several people that tested this out and it does appear you also get all Weapon Proficiencies with the exception of: Breath, Damage & Attachments.
Now, I'm not sure if it actually includes Proficiencies that were not originally selectable for the weapon you are using, but from what I have read, it seems like it does.
Glad I use the Specialist Strike Package for all my classes.
The M60E4 has the fastest time to kill within close range of all the LMGs. I've used this weapon for quite a while (2300 kills atm) and I say strengthen this gun's power points with Speed and Rapid Fire.
If you can get up close and personal, you'll outgun most SMGs assuming you use [Extreme Conditioning - Quickdraw - Stalker]. I also recommend running specialist [Steady Aim - Sleight of Hand - Dead Silence/Sitrep].
As for your medium range kills, 3 round bursts mitigate the recoil while still offering usability.
Lastly, for the longshots, play this weapon like a semi-auto assault rifle (3-4 shot kill)
It will take a considerable amount of time to get used to the *ungodly* iron sights, but with the majority of the maps being tightly-cornered and catered to CQC, there shouldn't be any big problems.
Hope I helped!