Proxies were design purely to use lower memory usage at rendertimes, and also to improve the viewport performance to an extent ( a fairly good extent mind you)
So, my rule of thumb is if you can render your scene without have memory problems, dont use proxies. If you are having memory problems, or foresee them in perhaps a huge scene then use them wisely.
As Percy said, one proxy per tree variety then instance those around. IF you just select all the existing trees in your scene and make one big proxy, then you will KILL the render time and your memory usage will be basically the same.
Opacity maps can cause problems, but if you drop the bitmap blur to 0.01 and turn of filter you will improve the render time quite abit from my experience. As for the refraction slot trick, I dont know if that holds any creditibility anymore.. but you could easily find out in a simple tests scene in a matter of minutes.
There are quite a few threads around for help lowering memory usage, including some good info on the spot3d pages.
Couple quick points, if you have displacement stick with 2d for lower memory usage in general, dont use HUGE displacement maps. And dont oversize your textures.