There has always been a bit of variability in these things even back to the P2 Vision+ which had a stated range of 800m. In a good environment like over water or open country my stock Vision+ has been out over 8500'...far in excess of the stated range. At my old house in a busy neighborhood with lots of trees I typically got 3000' or less unless I flew high enough to maintain a direct LOS between me & the P2.
I would first try range testing in an open environment like over water or open farmland. Are you losing control signal or video? Once you find a good place & get a few range-check runs, start on the simple side like a Windsurfer. <$15 & it just slips over the antenna. That would be good for the control side. Next up I would try something like the non-amplified FVPLR or DBS mods. Along the way keep testing at your known good site to compare results.
If you still desire more range you can add RC-side amps, & even a bird-side amp & antennas. At this point the whole setup becomes rather cumbersome. You have this big rig with all these cables & an external battery. If you do bird-side mods now your Phantom has antennas hanging out & a tray to carry the amp, etc.. The additional weight & power draw reduces flight times. Once you get beyond 3-4 miles out you are nearing the limits of battery power even on a stock aircraft.
So I recommend finding a good spot to determine your bird's stock range, then take it in increments until you get where you want to be with the least amount of additional equipment.
