The pre-order bonuses are unique items, with unique stats. And they AREN'T going to be unlocked through normal play.
The Soda Can/ArkTactical silencers probably have different equip speed and damage/accuracy modifiers compared with the regular silencer - they won't be better, they'll just alter stats in a slightly different way to the regular version.
And as for the bonuses not being released to other players, all that's been said is that SD "have no plans" to release them - this leaves them with the option of saying "well at the time we HADN'T planned to do it..." - it doesn't guarantee that the exclusives will REMAIN exclusive forever. Just that they ALMOST certainly won't be released straight away to everyone. I'm not saying they definitely will be, just keeping in perspective that they're not saying an unambiguous "We will never release the pre-order bonuses outside of pre-ordering" - they're leaving themselves the option of changing their minds.