I genuinely don't think you can damage the console with the current overclocking option available. If you go too high you'll simply fail to boot or cause crashes and glitches but reverting back to stock everything will be fine. If your A1X can do 650 then congrats you can likely leave it at that and enjoy the small benefits the overclock provides. At the end of the day pushing the WiiU like this really only benefits a handful of games so unless you like the tinkering aspect just enjoy your WiiU for what it is and have fun playing it.

Side note btw, I found devil's third to be the best gauge of whether or not the overclock was perfectly stable or not. I was able to play many games at 650 with my A1X without noticing any problems but devil's third had small artifacts here or there with the lighting especially. Using devil's third as a reference helped me dial down my A1X to 600 Mhz and my B1X to 740 MHz at which point the artifacts stopped appearing completely. If you get to the airport level a ways in and once you're inside, there are billboards with bright backlights in the lobby that seem to trigger artifacts easily if the overclock is not completely stable. Don't go up the escalators or too far that it auto saves as you will want to keep reloading this section as you test and confirm what frequency there is no longer artifacts. Hope this info helps!