you're just talking about RC control signal, not FPV right? with a helix receiver antenna, as long as your transmit antenna isn't terrible you should be able to go past 1.5km.
You DO always wait for a good home-lock before flying right? so even if you did fly too far and the P2 triggered RTH, as soon as it comes a few meters back towards you you can flip the toggle switch and resume control... it's not like you have to wait for it to fully RTH and land.
anyway it is possible to upgrade the transmit antenna on your RC since those stock antennas aren't that great... I've done this myself several times now the equipment involved is VERY sensitive it's super super easy to damage the antenna connector on the RC transmit board so you have to be extremely careful.
You have to be careful because the tiny little antenna connector is covered in sealant gunk which you have to remove without damaging the connector.
This video will show you the process of opening the RC and getting the gunk off the connector... you don't necessarily need to watch every second of the video because he sure takes his time... the gunk cleaning part starts around the 7:20 mark
Once you've got a clean antenna connector on the RC transmit board, THEN it's easy to put on a new cable which gives you a female antenna connector into which you can plug anything.
I personally put on
this cable and then
this half-sphere helix antenna from FPVLR, which I mounted to the top of my display.... so I've got that half-sphere transmit helix taped to one side, and I plug in (to my display) the FPVLR helix receiver antenna on the other side. both are directional antennas so I need to face the Phantom at all times.