In c1, bullet impacts from the scar for instance vanished after some time, which is, from my opinion a small but annoying detail. performance wouldn't suffer by including this small detail, would it?
I think it would affect performance eventually but I couldn't say by how much. I think the way it was handled in Crysis was pretty good, I never noticed bullet holes and marks fading away or even the fallen soldiers. I saw some footage of the 360 version and there was a scene where some bullet holes faded away after a few seconds which was very noticeable and ruined the immersion for me.
On PC there was a console command that made bullet holes last essentially forever (well over an hour) in Crysis 1 / Warhead / Far Cry 1. Considering that the console commands are coming back, it should be possible to do the same change in Crysis 2. As for consoles, their configs are hard-coded, making this change impossible.
In Far Cry i played very stealthy with a silenced MP5 trying to headshot the enemies from a far, and it was always a joy walking to the corpses and look for the bulletwounds because they were acurate and showed were I had hit.
Crysis and Wahread I think didn't have any bullet wounds except for the hat that some enemies wore. Would be very nice if this could make it to Crysis 2 but I believe the problem is the german rating laws for games.