Will the items in the Supporter addition be unlockable through gameplay? If not, are they purely cosmetic, or do they have elements that have an impact on gameplay?
They're exclusive to the supporter edition. The weapons themselves are not any "better" than the guns you get in the standard edition. However, there are some slight changes such as tritium sights for the FBI G19, and a wider viewport for the FBI Shield (by about ~10cm on each side or so). Both changes are marginal for gameplay, though they could technically give you an advantage.