Frankly, I doubt the parcel outlines are of very much use on the mini-map (too small, too inaccurate), but the banned parcels may help (even if they tend to appear very late since this is the server initiative to send the info to the viewer or not about them, depending on your camera movement and the risk of "collision" with the banned parcel).
Indeed, and I noticed this annoyance when I started motor-boating in SL, only three months or so ago (*)... Those tiny protected land/waterway corners between private parcels and those late appearing ban walls are really a nuisance, so I had to "find a way around them" (both a water way, and a smarter coding of the viewer).
Yes, the "Property lines at water surface" is the first feature I implemented and by far the most useful. But the better ban walls rendering in-world and the banned parcels rendering in the mini-map should help (and may even allow to disable the property lines display when you want to enjoy the 3D-world view without them to ruin it).
(*) I guess I don't get enough RPs any more in SL nowadays... so, I found other distractions. If you see a "Black Arrow" bolide at the surface of SL's seas, that's me !... A great way for me to stress-test the viewer too.