The weapons are already highly accurate with minimal spread and no recoil, and so therefore a scope would be pretty useless unfortunately. If recoil and spread were added to the game to allow for a scope, then Marines would be significantly under-powered before this upgrade was researched.
Comments
Also, models are removed at a certain range for rendering (client-side ; you can test that easily in crevice on any public server).
And I fear that if that gets implemented the next thing we get is a sniper... (dafuk).