I go down the street, there is a machine that I can fill up my 5gal carboys for $0.25/gal, so not even worth getting an RO system for me, since I only do a very small grow. You could buy a filter to bring down your PPM if you are worried, they can end up costing quite a bit more per gallon if you are using a lot of water and swapping out filters often. You can also just mix a couple gallons of distilled or RO water from the store (Grocery has distilled for $0.88/gallon here) with your tap water to bring down the total PPM. I am totally sold on the benefits of RO for hydro, for soil though, my tap did just fine.
As far as switching lights, I would have no qualms about extending my dark period one time 10 days in, if it was going to result in better temp controls. I think the one day of extended dark would be a minor bump in the road compared to better temp management. Now, if I was 30-40 days in.. it would depend on how much I would gain by changing the light schedule, if I had real temp problems I would probably still do it though.