The recipe looks pretty solid. For what it's worth, here's my experience:
I have brewed two sweet stouts with the same grain bill. One of which I followed the water advice listed here: http://www.homebrewtalk.com/f36/mash...-stout-173417/
, although I used a Chloride to Sulfate ratio of 2. The other, I brewed with straight RO water. The sweet stout with the water additions was the better beer. It was smoother, maltier, and creamier than the straight RO version. You may also have luck adding salt additions post-mash to alter the flavor of the finished beer while avoiding potential mash pH issues.
Water profile for the adjusted beer (values in ppm):
Calcium = 90
Magnesium = 17
Sodium = 95
Chloride = 138
Chloride / Sulfate = 1.92
Of course, there's always nitrogren too.
Full Disclosure: I am not an expert on water chemistry.