Nick, I agree you would have thought it would be a little quicker that 4.5 Secs is quick. The best I've had out of my Classic 3.9Ltr Plus 8 is 6.0Secs one day when I decided to go for it and time it.
I think if you want to achieve the catalog's measurement times, you have to rape the car extremely. I've been afraid of that so far.