Dan, I'm not sure how much you will lose in the cooking process but I think if you brine it before cooking you will minimize loss and improve flavour. I plan for 1/2 lb per guest (child OR adult) and you are riding on the line with this amount in my opinion. I don't know that I would necessarily add a protein by cooking a ham or anything, but I would perhaps tweak the menu a bit to make sure everyone is full. Here's my suggestion:
Appetizers: Shrimp ring and veggie platter (both economical and easy to prep)
Entree: Roasted turkey breast (brined to maintain moisture), either oven roasted potatoes or roasted root veggies (onions, potatoes, carrots tossed in 1 tbsp each oil, powdered beef boullion, balsamic vinegar) and steamed green beans finished with a pat of butter and a sniff of salt.
Salad: Make your salad pretty, theres lot of fresh veggies out there with lots of colour at the moment. The plates will have enough on them so serve it in salad bowls on the side.
Dessert: If you feel adventurous, bake a pie or pound cake (both easy and reasonable $) If you don't want to do that serve a fruit and cheese platter. More expensive but a lovely finish.
That should suffice for everyone. I agree with Patty about the soup. Too heavy on the poultry end of things. If you want to do a soup instead of any other kind of appetizer, perhaps choose something a bit different. A gazpacho or something creamy. (Potato soup?)
Good luck! Let us know what you chose to do and how it all went.
I would rather have MORE then enough then make my guests think they have to eat lightly so there is enought to go around. Leftovers are great!
Do your guests a favor and don't stuff them. 
