Would this give a large performance hit though? Never actually seen this implemented in a game before so I don't know

I don't think they'd have to do that - most people's complaints were purely that they looked goofy or clipped with eye wear rather than the fact that they are purely aesthetic - many things in this game exist for such reasons, including the OPFOR units cooling system on their uniforms - which serves no functional purpose.

I'm completely fine with having things in the game that don't affect gameplay, I'm not asking for a fully simulated HUD - I just liked how the HUD pieces looked :P