I put apples, herbs, and onions in the cavity so they would sacrifice their moisture instead of the moisture being drawn out of the bird. The more aromatics you put in the cavity, the more it will infuse into the meat. There wasn't a distinct onion or apple taste to the bird, but more a delicious sweet smoky taste. I'm sure it was the combination of the apple, aromatics, smoke, and brine, it all combined beautifully...I won't change a thing. You can think of it like putting a beer can up a Chicken's keester to make beer can chicken. It provides some moisture, and some essence into the final product.
If you do put a compound butter under the skin, it doesn't allow air in and moisture to escape...actually it would work opposite of that. The skin drapes on the bird and doesn't allow air under it. The extra butter (fat) bastes the bird and the herbs give a great flavor.
For your turkey, you want to stoke the smoker up to 325° or so because turkey doesn't have as much fatty
tissue as ribs or butts. By cooking them faster (not low and slow) you preserve the flavorful juice in the meat and don't dry them out. At 325F, 15-pound bird will be done in about 3 to 3.5 hours as a starting point...I think my bird took about 2 1/2 hours.
If your bird comes with a pop up thermometer, take that out and insert a probe into the front of the breast so the tip is in the center of the thickest part of the breast (usually right where the popup thermometer was). At 165°F in the breast it is safe according to the United States Department of Agriculture (USDA). From the USDA site:
minimum internal temperature of 165 degrees F throughout the bird
A whole turkey is safe cooked to
as measured with a food thermometer. Check the internal temperature in the innermost part of the thigh and wing and the thickest part of the breast. For reasons of personal preference, consumers may choose to cook poultry to higher temperatures. Turkey breasts must reach 165 degrees.
More than likely, when the breast is at 165° the thighs are likely to be higher than that. To ensure a moist bird, besides brining it, I recommend that you take it out of the smoker when the internal temp of the breast reaches 160°, 165° max. My rationale is this...If the temp is 160°F in the center of the breast when you take it off, it will rise at least another 5°F as it rests. Be sure to use an accurate thermometer (I use my therma-pen) to make sure you are good to go. Cook to temp, and not to time...this will make sure you have a fantastic bird for your special day. Good luck and take lots of pictures.
I also use this same philosophy for smaller birds on the smoker...they come out great too!