Nothing beats a fresh turkey. I have use the middle rack with beer can chicken and they came out great. A big turkey in the top rack will have the breast too close to the dome, and if it touches the dome, the meat will burn. Like I say, if I would make an 18 lbs, I would use the middle rack; keep an instant thermometer handy. If you have a probe thermometer even better.
I do place onions and celery sticks inside the turkey cavity. I have use the water pan with water and without, and I think it makes no difference on the taste.