Sorry, I should have been a bit clearer about what I was thinking. My assumption was that the eastern hills still will be a target, so they would have to drive around the other side of the rim to start heading southeast. This also assumes that they won't be able to drive straight through the crater (although I recall from a press conference that Malin is already advocating for getting down to the dune/drift materials on the floor). They will have to do more hazard avoidance if there is a larger population of ejecta blocks, which will add to the total traverse and certainly slow it down (<10-15 m per day ??). So for this case I'm sticking with a drive of 600+ m and probably 40+ Sols. Of course, just going to the heat shield and punting on the eastern hills is another scenario.
But since nothing is known about how well the rover will perform on traverses and we don't really know the nature of the surface at the rim, this is nothing more than a WAG. :-)