With these figures you have to keep in mind that the result is very much depending on small time variations. If you include a time for the brake system to apply full brake power of typ 0.1 to 0.2 seconds (as already shown before), a decelleration of -7.55 m/s^2 would not avoid the impact. In accident reconstruction these relations are shown in distance-time-graphs as you can see in the attached figure.
At those speeds 0.1 seconds, resp. 2 meters can change the result.
But of course, a prompt braking action of the autonomous car system would have reduced the impact velocity very significantly! (in this case and with -7.55 m/s^2, to approx. 15 km/h, see graph)
And yes, with good conditions higher decelleration rates are possible, but in real driving situations you would stick below 1g. For the case discussed here, you would have to perform braking tests at the accident scene.