The biggest killer in a single is the fact that there is little to restrain the massive block of the engine from coming back through the firewall into the cockpit.
Interesting. What data is the assessment based on? Since another prevalent school of thought suggests the engine in a single protects the occupants, due to the far stronger nose structure that is necessarily in place to support the engine during normal operations. In a twin the nose is a light and flimsy aluminium or plastic design with no protective ability whatsoever.
There should be ample data from crash investigations, and studies of that data, to suggest what the main causes of injury in light aircraft crashes are. From the data, it should be fairly straightforward to conclude what the most effective improvements would be.
So in discussions like this one, where is that data?
On the other hand, if light aircraft (passive) crash safety were mainly a sales pitch, the lack of supporting data would be understandable...
Regarding the comparison with formula 1, vertical impact speed is generally not a problem in formula 1 crashes. Reducing vertical impact forces, thus lowering the risk of crippling or fatal spinal compression injuries, is a large part of what modern increased crash safety aviation seats (in light aircraft!) are about. That is why we can't put our knees on the seats anymore, or put stuff underneath them...
Passive crash safety would be the last thing I'd be looking for in selecting an aircraft.