This is how you do it:
1. Find the file "ROGame.ini" in My Documents/My Games/RedOrchestra2/ROGame/Config/
2. Double click it, it opens in notepad
3. Edit the following under [ROGame.ROPawn]:
FXLifetimeMultiplier=1.000000 , this default setting results in a 30 second duration for corpses to stay. Change it to 2.0 and you get 60 seconds, and so on.
I changed mine to 20, so corpses stay 10 minutes.
FPS get lower to the end of the match. In Commissars House I actually notice the FPS drop, but most maps run fine here.