I think the whole unlock system is a cynical manipulation of us all and been an awful contribution to FPS gaming… but anyways. In general the unlockable weapons are not noticeably better than their standard counterparts in games.
Lots of anecdotal stuff about this being better than that etc however.
Anyway – if you wanna know the real deal I have found all the stats for the various weapons and attachments. Quite interesting – I have certainly used some of the info here to change my loadouts/ how I use some weapons.
Enjoy.
http://symthic.com/?s=bf3