It's a bit weird but that's what I've ended up after playing many missions in various terrains.
I've made some modifications so that NVietnam can operate SA-3, SA-6, SA-8 etc. Their performance proved quite deadly! Even the older SA-2F gives a Pk of at least 0.4 - 0.5 if flying above 1500ft without use of countermeasures. In general all SAMs fired their missiles when targets came within their range and most of the times they seemed to follow the path direct to the target.
In other terrains SA-2F will hit the target only when flying high and no hard manuevers. SA-3 will fire in closer ranges, SA-6 reacts very slowly and sometimes doesn't even fire! SA-8 doesn't react very fast in addition to Vietnam when fires all missiles if needed. MANPADS SA-14 and SA-18 usually don't fire to enemy aircrafts but in Vietnam don't just fire but are also quite effective when flying with no flares.
I've installed the terrain files from SF2V XP V2.0 and SAM pack.
I've also tested the Red Side SAM performance in a "clean" full merged SF2V install. SA-2F was as good as in the modded terrain.