Potential Wattage leading to damage

[charger] 5.1v 2.5 amp official rspry pi charger If i tried to connect a sim800l v2 directly to the raspberry pi 3 b+ would the sim800l v2 break from overheating or would nothing happen.

It depends if it is 5v or 3,3v ? - 5v, if the combined current consumption of the Pi and SIM800 exceed the fuse in the Pi it will break. - 3,3v, if the power consumption exceed the (~500mA is the number that I have seen on other sites, not a official value) that is available from the Pi.