The one's you drove may have felt tired (which I suspect was not the case, more a personal feeling, certainly not with 5K on the clock unless they had spent it completely on track) but I can be pretty confident they'd be bang on the numbers at worst. My experience showed me that a 911 needs maybe 12K on the clock before it comes into it's own and I suspect with 5K on the clock it would still be a little tight. My GT3 wouldn't tick over for the first few days I had it, kept stalling annoyingly. The perscription was a long Italian service and bingo, prolem solved. These are cars that are built out of stone in the main, they will take a good kicking and in fact respond to it too.
I always felt (baseless as it turned out) that my E46 M3's were about to go pop as I got close to to 8K rpm where as the 911 never once felt anything other than rock hard. They feel unburstable the 911, you're going to love this I suspect and if you do you will become beholden to Pork even if you have nice things in between. Tis a bug I'm afraid, a yella one.