For your Audi, according to the online maintenance schedule, your spark plugs should be "replaced at 35,000 miles or 3 years, whichever occurs first. Then, every 40,000 miles or 3 years, whichever occurs first." I'm guessing, based on the year of your car that it's close to 100K, so it may be time for a tune up?
So, if you're running a little rough and mileage is dipping, I'd look to replace the plugs AND your coil packs. I used to have an A4 and I had a rough idle and a couple of the coil packs needed to be replaced. Not cheap for the parts, but you can do it if you have a socket wrench and extension to save on labor. Take a peek at one of the Audi forums, I'm sue you'll find the exact info you need.
As far as answering your original question: if you pour the whole can of Seafoam into your gas tank, it will not blow up your engine or make it die. Follow the directions on the can and you'll be fine. Just don't expect it to fix it if you need scheduled maintenance.