I know several people who feed all wet food diets. For the most part I hear that wet food does not harm teeth anymore than dry. While I wholly agree that dry food does not CLEAN teeth (at least not where they need to be cleaned, up against the gums), I am not totally convinced that there is not some benefit to the chewing a cat does for a strong jaw. I am also NOW completely sure that dry food is not all that great for cats and that a wet food diet, even if it is worse for teeth, is way better for your cat. If teeth worry you let your cat have some real raw meat (not ground, go with chicken meat cut into cubes, chicken wings or chicken necks) a few times a week. That way the cat gets a cleaning and gets to use those jaw muscles. Of course when first introducing the cat to the raw meat start small with just small pieces of the chicken meat, when he learns how to chew move up to bigger and bigger chunks.
*Supervise all raw meals especially at first, the cat may take some time to learn how to rip off meat and chew meat and bones, if bones worry you (thought they are pretty safe if RAW) then big hunks of meat can still do a nice cleaning job. Stick to doing this about 2-3 times a week, if you overdo it without actually feeding a raw food meal (with all proper nutrients) you may be messing with the balance of the cat's diet*
Check out this site (I'm showing it to everybody because I just discovered it, and LOVE it): http://www.catinfo.org/
Also forgot to answer your other questions: If I fed my cats all wet food they would each eat a 5.5 ounce can a day. that would be the downside to wet food, it's expensive. Prey model raw actually ends up being cheaper for most people...
If you can't go ALL wet then do part wet, as much as you can, it will help!