I bought my P5 off a friend that bought it new. Right after he bought it, he put Pro Armor 4-pt harnesses in both sides. In the Fall that year we took the P5 on our annual out-of-state ride to South Dakota. The P5 was used by another friend's nieces, so that was good to have 4-points on them in the mountains. The following spring I bought it off him so he could go get a Talon. I drove the P5 3 times on trail rides with the 4-pt and yes, it did make you feel safe. But like Trigger pointed out- your movement is restricted.
The following year I ended up taking the 4-pt out and putting the lap/shoulder belts back in, mostly because we started riding as a family and the youngest's car seat was easier to put in the passenger seat with the factory belt. As well as my wife wanted nothing to do with the 4-pt belts.