You are lucky it makes an enormous difference. I live in FL, and fly out on the beach, and inland. Up the beach, even with all the condos (the interference reaching the waterline must be minimal), I've seen 12,000' (Stock. P3P Weak signal prompted turn around) but agonizingly (for me) I haven't gotten to try my P4 out in this clean an environment. This is at altitudes below 200'. LOS when it's that flat is easy.
In land a bit, where I'm living now, I've seen ranges in the 2,500' range, in areas with large housing developments, at high enough altitudes where I know I'm still LOS not quite 400', but high. BIG DIFFERENCE. I'd say some of these areas really shouldn't be over flown. You can try and not be directly over dense tracts of houses, but it's hard. Obviously interference.
FOR MY P4: In directions where it's relatively non-residential, and no neighborhoods are ever below, I saw 1.5 miles (about 8,000'). And I was naughty. I had to go up to 700' to get there.
If there was little interference, why so much more range over the beach (12,000')? I'm figuring a little interference, but 700'/8000' will show an angle of about 5°. Pretty shallow. Was I really LOS? Probably not quite. Plus we are talking P3P vs P4. But over this same route I saw about the same with the P3P.
That's my range info. The 12,000' over the beach flight, was when I was really new, and was downwind! What an idiot. Only slightly downwind, but still... I got back but it died when I catch landed it -- ready to auto land in seconds.
LOS can be REALLY important. At 2,200', I was getting a strong signal at 200', and dropped lower to look at something. This area was modestly industrial, very little population. At 150', I lost the signal like a switch had been turned. I think I dropped below a wide concrete intervening building. It faithfully flew back and I quickly got the signal back.
But I learned that in anyplace with tree coverage, once you're 2,000' away (or less), you aren't going to get near the ground and get a signal. Clear LOS is BIG. HOW MUCH THESE ANTENNAS AND/OR BOOSTERS CAN HELP WITH THAT ("PENETRATATION") I'M CURIOUS ABOUT.