The Bosch Aquastar 250SX is the one I was thinking of getting, but your comment about the single lever faucets is something I did not consider. Would the problem be solved or alleviated by lowering the temp on the hot water to use proportionately more? What temp is yours set at?
Also, you said you used the existing flue for the exhaust. How did you keep it centered and what did you use to seal it off? Mine is 5" direct vent pipe, curretnly zero clearance to combustibles. I do not know what clearance would be appropriate for the 3" stainless inside there. I just ordered an infrared thermometer and will check the skin temp on the current outer flue. I would have to drill a hole in the outer pipe (then seal it) to get a reading off the inner. I believe the stainless is up around 450 degrees.