How much higher gain antennas, or an amplifier, help with range depends on a lot of factors. It isn't generally true that an additional 6 dB antenna gain will give you double the range. Let me explain. (I have a P3 4K but the same applies to the P3 Standard.)
First of all consider the 5.8GHz control signal side. This is one way from the RC to the drone. So your control range depends on two things being true: there is enough RF power from the RC reaching the 5.8GHz drone antennas for it to detect the signal, and the noise floor at the drone is low enough to decode the signal. There is nothing you can do about the noise floor on the drone side (without modifying the drone) - the drone has omni antennas. So it is really just about the RF power at the drone. There are two ways to increase this - add antenna gain on the RC side, or add power (using an amplifier) on the RC side. In both cases 6 dBi gain (either via an antenna or via an amplifier) will roughly double your control distance.
On the 2.4GHz video downlink side, things are more complicated. This is one way from the drone to the RC (not technically true, but good enough for this discussion). Again the video downlink range depends on two things being true: there is enough RF power from the drone reaching the 2.4GHz RC antennas for it to detect the signal, and the noise floor at the RC is low enough to decode the signal. In this case, there is nothing you can do to change the RF power from the drone reaching the RC. An amplifier will have some level of receive gain (usually 12 dBi or thereabouts) but this will amplify the noise as well as the signal, so amplifiers will only help if the signal is low but the noise is also low. This will typically be the case if for example you are flying in a rural environment with very little ambient RF noise, but in an urban environment where you are noise limited an amplifier will likely not help at all. Note that the transmit gain of the amplifier really has no impact here, because the video downlink is from the drone to the RC. This is why people see very little difference on the downlink side whether they are using a 1W or 4W amplifier. However the antenna gain is important. A 6 dBi antenna gain means not just that the transmission cone is shrunk by 4X in volume (so you get 4X more power at a given point within the cone - i.e. twice the distance - vs. an isotropic emitter but again this isn't important for the video downlink) but also that the receive cone is shrunk by 4X in volume. This means that (assuming the noise is isotropic, i.e. equal in all directions) your received signal level won't increase, but the noise level will drop by 4X. So again if you are flying in open environments, a 6 dBi antenna gain may double your video downlink distance. But in an urban environment, if the drone is flying near a bunch of RF sources (like wi-fi hotspots), if you are aiming the RC towards the drone the antenna is also capturing the RF noise from those hotspots. So the noise level may drop by much less than 4X, or not at all. So you will see much less than a doubling in distance.
So in summary: for the control link, 6 dBi on the antenna or the amp will usually double your distance in any environment. For the video link, 6 dBi on the amp will probably do very little, and 6 dBi on the antenna will double your distance in an open environment but may do very little in an RF-congested (urban) environment.
My own set-up is a Phantom 3 4K. Stock I could get about 2000ft in an open environment. With the ARGtek mod (two 7dBi 2.4GHz panels, one 10 dBi 5.8GHz omni) I can get about 6000ft in an open environment. With two 3W 2.4GHz Sunhans amplifiers and one 4W REXUAV 5.8GHz amplifier, I can get about 17000ft in an open environment. So the amplifiers help a lot, because I am signal limited rather than noise limited.
In an urban environment the distance gains are much smaller. Stock I could get about 1000ft, with ARGtek around 2500ft and with the amplifiers around 3500ft. So the amplifiers help very little, because I am noise limited rather than signal limited. But it's very variable.
The great thing about the ARGtek set-up is not necessarily that the ARGtek antennas are particularly great, but that without destroying the controller you get 3 SMA antenna connectors that you can place whatever antennas you want on, so you can very quickly do experiments. I've ordered a bunch of different antennas and will be testing them at some point to see how they work vs. the standard ARGtek antennas.
Sorry for the length of this. It's a complicated topic. Hope this helps someone.
-Adrian
