I recently maxed out my class with the specialist kill streak in MW3. When I got 2 kills after my last perk, I got a "specialist bonus". Afterwards, I appeared to have sleight of hand(which wasn't one of my specialist perks). So what exactly does the specifialist bonus do? I've done this before but never noticed a real change and figured it was just for points. I didn't noticed I unlocked more perks
The specialist bonus is given when you achieve an 8 kill streak with the Specialist package (or 7 kill streak if you have Hardline).
It gives you all perks that you have unlocked (even if you have not chosen them as part of your package, and excepting Overkill), and all weapon proficiencies (except Impact, Breath, Damage, and Attachments). Check out the COD wiki for details.
What commando said except it gives you the perks you have unlocked. e.g. if you got pro on everything you would get the pro effects, if you got pro on nothing you get no pro effects.
Also, you get bonus xp for each kill once you've earned the bonus, till you die Getting the specialist bonus also gives credit towards the "OMA" challenge. As additional trivia, the specialist bonus icon resembles the "One Man Army" perk icon from MW2.
The above true except that you unlock all perks full stop regardless of whether you have unlocked for example if you use specialist at lvl 20 you will achieve scavenger when you get the bonus even though you have yet to unlock it.