I read his tweet to mean that they may be able to further engineer the Cybertruck for even better Cd, if they wanted to. And not that the drag coefficient could be further improved by owners. It's rare there's much owners can do in terms of actually improving drag anyway. Almost all aftermarket / DIY stuff is going to increase drag, not improve it. Aftermarket vendors usually don't computer model and/or wind test their stuff. Nor does the average owner careI wonder what he means by “extreme effort?” Is this things that have already been done (mirror delete, flat underbody) or does he mean that owners can tape the panel gaps and add saucer hubcaps to get to .30 Cd?
For what it’s worth my aerospace engineer friend says that abrupt angles like the top are bad for drag, but maybe Elon has some tricks up his sleeve to “trip” the airflow.