I'm not entirely sure but I would say that 0-60 times would fall more under a cars torque specs as opposed to its hp rating so maybe someone should factor those as well... i'm too lazy too..
There are multiple factors involved in any 0 to x# run other than just hp/torque... i.e. driver, elevation, temperatures, weight, tires...the list can probably go on and on.
Regardless, the highest has been 5.0 and the lowest 4.6. Actual is probably somewhere between them which is still sub 5 and pretty respectable in that time and age. Either way the 911's weren't drag cars so who cares what it's 0-60 time was. Even if it had been 10 secs, it's looks and other performance aspects for the day were killer. It's simply a desirable car..then and now
blue....in regards to the 964 bumpers, I would have to say it was for weight saving and/or maybe simple aethestics to updates it's look. Those old 911 bumper are pretty heavy...just getting rid of the huge 911 rear bumperettes probably saved 20-30 lbs in weight savings alone.