You aren't accounting for driver weight though and also not sure if the kerb weight only includes a certain amount of fuel, spare wheel, etc.
Also peak bhp is only a guideline. It's all about 'the area under the graph'. A turbo car will always hav more low-down torque than a NA car which both have the same peak power. The turbo car would be quicker. And there's also the increased drivetrain losses of the Quattro to consider.
Leon (Zerok66) managed a 12.4 in his 500bhp S4 and I think JohnnyB (JJB Motors) has just matched that in his 500bhp S4 the other weekend at York. I've yet to see someone in the UK crack 11's in a B5, and there are some high powered machines out there.
The lets torque calculator seems to apply a constant for driver weight (it specifically asks for the weight without driver), but lets be honest, 20-30kgs here or there isnt going to make squat of a difference when you've got 400hp on tap.
Drivetrain losses are pretty much irrelevant in real life. They appear hugely different on a rolling road, but thats only becuase your not measuring the losses from the two undriven wheels on the 2wd car.
Can you tell me what your cars sticker says the weight is, and if you've done any mods that might have increased it?
The site claims 1700kgs for an RS4 and 1505kgs for an S4, but that seems like a large difference for what is in effect a pretty similar car.