According to what I've read, Stasis tune will void your Audi warranty, however Stasis will assume the remainder of your warranty so essentially, you wont lose it, however people have been screwed by it.
Audi can, and does, hook up your car to the computer during most of the services (not oil change, unless there is a reason to do so) and it will flag your car TD1 (volkswagon flag of a modified car). Audi *can* deny you warranty. My service advisor told me that is only for an engine tune. If you put on an exhaust and your suspension fails, he said they wont deny warranty.. but... YMMV.
The salesman is trying to make the sale, and anything he says, unless written on the bill of sale, should be taken with a grain of salt.