I'm not qualified to talk about D3 vs C6, but I agree with Mike on D2 vs C5. I regularly drive my daily C5 A6 to Leeds (300 miles) and tend to stop half way. If I'm in the D2, I can drive until it runs out of fuel without having to stop. The A6 is an appliance, albeit one which is quite entertaining to hoon around the countryside. Every time I sit in a D2 though, it still feels special.
Re engine choices, the 2.7TDI is just a watered-down version of the 3.0, I think for company car tax reasons. Its just a bit rubbish and doesn't really have anything going for it - same maintenance costs as the 3.0, much the same fuel consumption, but significantly less power and less tunable. Also some weird bespoke bits like the turbo and injectors which cost considerably more to replace than the 3.0 versions because there are fewer of them about.
Afaik the A6 Le Mans is a specced up 3.0TDI quattro S-line. The harsh ride comments are probably because it has quite large wheels as standard - 20" I think. People shopping for an A6 may not be used to the ride a 35 profile tyre gives you
