I think there is only one easy way to be sure you are having less impact on the environment due to your purchase and use of backpacking gear - purchase and use less of it. By using less, the uncertainties of the relative environmental impacts of synthetics vs. natural fibers, recycled vs. new, and so on, become less critical. It goes back to the basic idea of Reduce, Reuse, Recycle in that order of importance.
So one aspect of traveling light in the backcountry - not taking unnecessary gear - is inherently a good idea for the environment, as long as you also don't buy that unnecessary gear. In other words, the footprint of your gear is not what you carry but what you own - and to a lesser extent how long you own it. Wehn you use something until it is worn out, you get the most use of the resources that went into it. When you get something new just because it's a little better and the old item sits in the closet, that's more footprint for effectively the same use. I think that if you must have new gear, it's actually better to sell the old stuff so someone else can use use it and wear it out, thus getting all the use out of it.
But of course it is more complex when looked at over the long term. Someone mentioned the closet full of differnt sleeping bags for different conditions, adn so on. If you have several different bags (for instance) and you use them all enough so that over a period of many years you wear them all out, then you would have worn out several bags if you had owned only one at a time - and would not have enjoyed the benefits of having the right bag for each trip. Complexities do indeed abound.