mantis,
Just read your opinions on the S5, agreed with most of it with the exception being the power but then later on noticed that your 335i was chipped which would explain a lot.
I read your opinion on the handbrake and agreed with this which is why when I ordered mine the dealer recommended the 'hill-hold' option which when you start your car up you press the automatic 'hill-hold' button which applies to handbrake every time the car comes to a total stop and automatically releases as soon as you pull forward or back. It's a great option which more cars should offer.
The steering is light and to a non-Audi driver will feel strange but after a week you grow use to it and it does firm up with speed plus it's quite accurate, it's not a Porsche but then again neither is the steering on a BMW. The only real disagreement I see is the power difference between the 335i and the S5, you said yours isn't stock so I can't comment on how much faster your car is than a normal 335i but I know that the S5 is quicker than one of them, admittedly not but much but it would be equal to about 2~3 car lengths by 100mph. Here's a review which seems to bare this opinion out.
The mistake most people make is turbo cars feel quicker because of the boost which isn't as linear as a big N/A car. The S5 is plenty quick enough for most people.