It is the same archetecture, just different clock speeds (pro and xt). Basically you can flash it if you can oc to the speed of an exactly the same archetecturally designed pcb/gpu. The thing is, lets say u do get it oced to xt speeds. If you can get it there on the clocks by ocing it, then its exactly the same as flashing it. Basically in that case, flashing would only make your default clocks higher and yield no performance gain sense its just a default clock speed change. (it also enables overdrive in the driver but thats useless if you are already at your card's limit)
My advice: oc your cpu and ram higher if you have room, oc your card up to its max, set the best ram timings and maybe even consider additional cooling if you arent cheap like i am
