First let me say I mis-spoke about the output BTU on the unit I have. I went to the basement this morning and checked it. I have a Rinnai model R-85 which has a maximum BTU of 185,000. I'm not sure how I put in that 285,000 BTU in my original post, but I've changed the original post to reflect the lower BTU.
At a flow rate of 6.5 GPM it will raise the water temp 50 degrees. Bear in mind that at a lower flow rate [I.E. one outlet open the flow rate is lower so the water temp will be higher, up to the temp set on the unit] you will be able to achieve max temp per your settings on the unit. Most units are preset at 120 degrees, but can be changed. My unit is set to 120 degrees. The other thing you have to keep in mind is when using hot water, do you really use just hot water? Most of the time you will be mixing cold with the hot, so the water flow through the unit will not be the water flow coming out the faucet or shower head. Even when filling the Jacuzzi tub we don't use straight hot water.
http://www.takagi.com/index.asp
http://www.rinnai.us/icms/rinnai/content/
Above are a couple of links to the units I looked at before I purchased. I got most of my information from the Takagi web site and was ready to purchase one of these, but I could not find a Takagi dealer within 100 miles of my home, so I went with Rinnai. These sites can answer most of your questions about tankless units.
Just as a side, when I looked at my unit it has the yellow sticker showing average cost per year. My unit costs, on average, $167.00/year to operate. In comparison my 40 gallon natural gas tank heater costs, on average, $257/year to operate. So there is a savings, but what I like most is that we can use both showers at the same time and not run out of hot water.
Karl