HIking in narrow canyons or rock scrambling with 5

Hi Folks,
I have a Fenix 3HR which I use mostly for hiking and rock scrambling. What I noticed with the 3HR (and many others as well) is that the GPS has terrible accuracy (milage is off by over 15%)unless you are in somewhat clear terrain. I have compared mileage with my Fenix 3HR and vivoactive, GPSMap 64s and other devices to confirm my suspicion.

Before I splurge for a 5 .. can anyone let me know how the gps/mileage accuracy on a 5x in canyons or scrambling situation?