Welcome along mate.
You're not really comparing like with like mate - you can't expect a big diesel estate to feel like a tuned hot hatch. There will be obvious differences in the charachter of the cars, the 535d is a very competant machine though, and as you need an estate a good compromise, mapped it will be putting out far more torque than the s3 and in the mid range at least should be as quick.
If you want to compare like with like the baby BM is the 1M coupe - that is a full second faster to 60 than an s3 at 4.7 seconds and runs circa 340bhp standard, with two turbos it also has massive tuning potential. SO I guess if you want a fair comparison then drive a tuned one of those.