the M9 (knife) has increased fire-rate, which I can only see in the stab action on the M9 Pro. The M9 vet didnt seem to be much different though, so those could all be cosmetic.
The m92 (pistol) has increased draw speed at the Pro version. I do not know about Vet or higher versions. The gold and silver m92 has increased power, the SE with faster reload, and now these "upgrade" with faster draw, but with no relation to one another.
the backpack is purely cosmetics, all are just +1 slot as far as i know (Veteran)
the black cap pro gives +5% sprint stamina (which isn't noticable) and 3% headshot protection (which is uselss). I do not know if higher ranks has bigger gains or additional stat bonuses.
best person to ask for these is greatful since he's the closest to getting the m9 champion lol...but no one will look like they'll get the m92 champion or the m16 champion since those are for ranks that we do not have (yet)...
*note, probably spelling errors and slight mistakes, i'm too tired to revise