I personally don’t think it has to do with Under Armor but more with layers of the correct material and what areas you protect. There are 3 key areas you must protect or you wont last in the woods on a cold day. In no particular order, Head/neck area, hands, feet. If one of these areas gets cold, you will be down out of your stand quicker than anything.
-For the feet, wool socks are awesome, they wick away moisture and keep your feet warm. Good boots are essential too. I like a boot with 1000 grams of thinsulate and waterproof. I hear a lot of people talk about putting hand warmers in the toes of their boots but I don’t see how that works if your boots fit your feet, where is the room for that to be in there, plus they need the oxygen to keep going, there isn’t much in the boot and the hand warmer will use it up in a heart-beat and you don’t get the full life of the hand warmer. Buy good boots and socks and your feet will stay warm.
-Head/neck area is crucial as this is where a lot of your body heat escapes from. A good wool stocking cap is a great way to keep heat from escaping your head. Couple that with a fleece face mask (I have one that goes over my entire head like what you see firefighters where at times) and you have a solid foundation for your head region to stay warm. But don’t stop there, if its really cold, make sure you have something around your neck as well, its another area that’s usually overlooked where heat can escape. A good mock turtleneck shirt is a good way to retain heat. This part is great if you are gun hunting, a small rolled up towel wrapped around the back of your neck and sides is a great way to keep heat in.
-The hands are a must obviously, and we will go back to wool again. A good pair of wool gloves with the fingers cut out of your release hand along with a hand muff tied to your waist is a good way to keep them digits warm. You can stuff hand warmers in your hand muff for instant warmth on your hands.
But to answer your question regarding bulky clothes and being able to shoot well, just do layers of the proper clothing. And like I said before, I don’t think Under Armor is the answer, yes it is thin and helps retain heat but for the money, I would for a different approach personally. Along with what I said about head, hands, and feet, I would go with a good set of polypro long underwear for starters, I would then put on a pair of fleece pants and my pair of cold weather bibs. You can always tuck one or two of those body warmers down your legs if you really need to. Now here is where you can save some money and loose some of the bulkiness on the top half of your body, where shooting your bow counts. After you put your polypro top on, grab a thermacare heat wrap and wrap the areas closest to your kidneys with it, its Velcro type front will allow you to adjust to get it right where you want, and they are inexpensive. Now you can throw on your mock turtleneck shirt, a fleece pullover (quarter-zip style) and then your outer jacket. You have very minimal clothing on with a lot of ways to retain heat on your body while giving heat to a core area of your body at the same time.