You must be up late David. I've just been synching my 13's starter valves, now I know what the famous 5-way connector is!
Too much rebound does give more or less the same effect as topping out, namely that as the wheel drops to follow the road (either after a bump or when a hollow is encountered) it can't quite go fast enough and so the tyre leaves contact for a bit. You can also get packing down, where the suspension gets compressed by a series of bumps but can't extend fast enough between them so it gets progressively more compressed. Any time that happens will be felt as harshness and loss of traction. The converse is if there is insufficient rebound, then the suspension extends too fast and the chassis starts to pitch and weave, a feeling of looseness, but the tyre stays in contact.
Too much rebound does give more or less the same effect as topping out, namely that as the wheel drops to follow the road (either after a bump or when a hollow is encountered) it can't quite go fast enough and so the tyre leaves contact for a bit. You can also get packing down, where the suspension gets compressed by a series of bumps but can't extend fast enough between them so it gets progressively more compressed. Any time that happens will be felt as harshness and loss of traction. The converse is if there is insufficient rebound, then the suspension extends too fast and the chassis starts to pitch and weave, a feeling of looseness, but the tyre stays in contact.