I don't know if I follow your first question correctly. Increasing the magnification on either a SFP or FFP scope will not alter your POI versus your POA. All it does is make the target you are shooting at appear larger to your eye. Just like a camera, when you zoom in, the subject gets larger.
The main benefit of an FFP over an SFP is that it is easier to use the reticle to range a target because the reticle in the scope grows as you increase magnification. 1 MIL/MOA is 1MIL/MOA at all magnification settings. For example, the reticle will be very small (probably will only barely be able to make out the small lines) at 4x, but at 16x the reticle is very large and very legible. This is a benefit because one MIL or one MOA (depending on the scope you buy) is always true, so it benefits in ranging a target. The negatives is that the reticle is small at lower magnification settings.
The main benefit of an SFP is that the reticle is a constant size and only the size of the target grows as magnification increases. There is only one power where you can range MIL or MOA 1:1.....which is usually the highest magnification the scope has. Ranging a target on lower magnification introduces more math to the equation because you will have to multiply your MIL/MOA estimation by a certain amount. Plus, most people are very familiar with SFP scopes because that.s what we all learned to shoot with.
I know this sounds kind of confusing, but once you get behind one/both types of scopes, the purpose of each become very evident.
I personally feel that if you always shoot at known distance targets, an SFP scope will suit you just fine. If you have a laser range finder and use it to range everything, still an SFP will be just fine.
If you shoot a lot of unknown distance stuff and use the reticle to estimate range, a FFP scope would probably be a better choice.
I kind of wish there was a flash video showing the difference between the two type of scopes because this is a very common question.
