and probably not solar powered, either. A blast like the one you describe would throw a lot of debris into the Martian atmosphere. Between the low gravity and the winds and depending on the size of the impactor, the debris could rain down a long distance from the impact site. You'd need to keep the rovers a safe distance from the impact site, and that distance would likely be at least a few miles. First, there's always a degree of uncertainty for the impact site, and winds can change. You'd want the rovers to be upwind of the impact site but if the wind shifts, you could be screwed, especially if your rovers are solar powered. If you want the rovers to reach the impact site within a few hours of impact, then the rovers would have to be able to move hundreds of times faster than the current ones. Of course, you wouldn't have to go all the way to the impact site - the debris itself would be useful to study. Still, the lightest debris will likely travel the greatest distance, and some of the deepest debris may be the most important to study.

As for orbiters examining the debris plume, that's a good idea. The timing could be worked out well in advance to ensure the orbiters will be in the right positions to see the plumb. However, you also have to make sure they carry the right type of sensors to be useful. Off hand, I don't know if the current orbiters would be able to take the kinds of readings that you have in mind.