I think it's been really worth the hours spent. Very nice render. I like the mist too, and the play of the different hill reflections in the water. If it renders so fast you can easily try some adjustments.
1. Soft reflections may indeed be interesting; use a reflective shader after the water shader (turn off reflections there) and set it to 0.004 to start with. If you go higher, you may need to increase quality. A little patchy wave action could be nice, just some whisps fo wind across the lake, use the patchiness in the water shader for that, you can test distribution of waves if you set the waves extremely rough temporarily.
2. I agree about the big tree, just a tad too obtrusive, IMHO. Either replace it by a thinner (green) tree (birch?), or move it to the right, so it sits in a grassy patch.
3. I would also look at the vegetation among the trees on the right far hillside; they seem a bit lighting up, so maybe too much translucency.
4. Also, increase Enviro light a bit to get a bit more light in the shadow parts (1.25 or so might work enough).