Good timing. I'd been doing other things, but I'm just about to start up again. I'm still trying to figure out the best solution for the particular tree I'm doing. It has clumps of blossom, so it's a liitle trickier than leaves to get looking good.
I took a look at some of the assets in the marketplace. I'm sorry, I can't remember the exact numbers, but if you're going to use many trees in a scene, as few polys as possible. It's still not at the stage where you can go crazy. If it's a hero tree, then you can up the number. Export is just FBX of the whole tree.
In the end, I decided to just get the trunk and main branches out of GrowFX and model the smaller branches, leaves and blossom seperately, although you could do them in GrowFX too. The poly count really jumps if you do smaller branches with meshes, even if you reduce the detail. The solution is to make some variants as meshes, bake them onto planes and position them as naturally as possible. Not GrowFX, but here are some of the links I'm using as reference. Also please let me know if you find anything useful!