Sorry but I would have to disagree. Seeing a fighter-sized plane with your bare eyes at 2km and especially 5km altitude is almost impossible unless you have a super vision, let alone knowing exactly where to look at, at the specific point in time.
Since this is a simulation, if those MANPADS are operating alone, they shouldn't be given any benefit of the doubt and be so deadly at the above mentioned altitudes. It's a science fiction and it shouldn't be there.
Unless they have a supporting logistics, IMO the only time they should be able to knock a (fighter) plane off the sky is if it's flying low enough for them to see it.