On the official DJI's website, P3 Standard, Advance, and Professional, CE mode is 16dbm and FCC is 20dbm.
If you force the EIRP to 27dbm you have to consider for re-match your antenna using SWR meter. If not, you will burn your tx module soon. Maybe not at current flight, but soon it will be.
16 dbm = 40 mW (CE)
20 dbm = 100 mW (FCC)
27 dbm = 501.2 mW (1/2 Watt!!)
The heat produced by internal tx modul will effect with many different behaviour to your unit. Could be strightly burn the tx modul, or might be started with slightly by damaging other moduls besides; like disconnected WiFi, decreased tx quality, slowly reduced video quality, etc.
If you all insist using 27 dbm with no re-matching antenna, then I hope everything will be okay for long lasting. At least, always set the home point before long range flight with 27 dbm.
So i need to get a 27dbm antenna if ill go with 27? Might sound stupid, but i know nothing about this. Batter safe than sorry imo