I have used small amounts of my own habarnero vinegar in my brine many times with fabulous results. Maybe 1T to a gallon of water.
Also, for a whole chicken or turkey I never leave the cavity empty, but rather stuff with chopped aromatics, and herbs. This Tgiving I added chopped apples, and some pieces of old bread that I soaked overnight in white wine. That helps make sure it doesn't dry out from the inside and adds great falvor. I don't put stuffing that I serve in the bird, but it's ok to if you make sure it comes to 165 degrees, either inside the bird or in a baking dish.
It is quite true that very acidic marinades will actually toughen meat. I think this is particularly true with poultry and shellfish. I have posted this article by Shirley Corriher several times; it's worth a read to understand the science behind marinating: http://www.taunton.com/finecooking/pages/c00157.asp
I have also posted a few articles about umami. Umami, specifically, is considered your fifth taste sensation, described as savory. It's conveyed by different foods, but primarily glutimates. MSG is one way to add umami, but naturally fermented foods like soy sauce, fish sauce, parmesan cheese, miso, etc. also add it. Subbing soy sauce for salt in things like salad dressing, soups, stews, marinades, etc. are a good way to heighten the savory flavor of food. It's a little known professional chef's trick.
Soy sauce in your brine mixture adds some umami. Sugar does not, but does work to enhance the savory flavor of the brine by cutting the harshness of the salt. Sort of like adding a pinch of sugar to a vinaigrette to cut the acid.
Here's a few links to umami:
You can google umami for tons more info.