I just don't see the value.
You could get a reference R290 (non-X) for ~$400 USD (if its near MSRP in Canada, as it is here where I am). A cheap AIO water cooler, $6 VRM heatsinks from ebay, any 120mm fan.. and have yourself an uber quiet and cool miner that pumps out 830 to 845 kH/s at stock settings, or 900+ with a small OC to 1ghz core and vram to 1350 (Hynix is easy, but Elpida is le-shiat).
So for the price of <$1,000 USD, you can have 1.7 Mh/s with 2X R290 Ghetto Cooled.. the way I see it, you cannot beat that value nor the silent operations nor power efficiency for the price.
As to your question about ROI, nobody can be certain, we can only deal with trends. Currently the trend is very good.
Edit: Z15CAM, I dont think it will be an issue with the 850W, under water they consume much less power than running at 90-95C.