If the OEM has accounted for the external gain properly in their calibration data (both board files and ART), it should work transparently for ath10k. The more you go from big/ reputable OEM, who knows how to do this properly (and to get the necessary certifications) to quick-hack designs using external frequencies shifters or power amplifiers, the more likely to enter difficult territories.
And yes, it possible to damage the hardware by running it outside of its specifications. If the OEM did their job according to the SOC vendors requirements (providing proper board files and ART), setting out-of-spec signals shouldn't be possible - if they didn't, anything can happen (this is often a case for vendors who use WLAN chipsets for non-wifi frequency ranges (e.g. 900 MHz) using external frequency splitters and only accounting for this in their custom (shim-) driver interfaces (unbeknown to the kernel/ wireless driver)).