It shouldn't matter much if you don't have higher straps. If your memory is clocked higher than the highest frequency strap, it will just use that one. At least it should. Now that I think about it, my Gigabyte would insta-crash if I clocked my memory to 2001, until I added a 2250 strap. Dunno wtf was up with that but that's not what's supposed to happen.

The real downside is that your card's default mem clock will end up using your experimental timings and you might keep soft bricking your card.

For undervolting the core you can try setting the voltage per dpm state but it probably won't go lower than 950mV without identifying the controller and adding some shit in the VOI table. Can't help you with that though 'cuz I haven't gotten to that point in my own modding yet. Still fighting with a damn Armor card with MJR to stop giving incorrect shares at 2150 I think I have to give up and go down to 2100.