13 GPM for a shower is a bunch of water and one really expensive shower but if that is what you enjoy go for it. Years ago I used the old style Paloma water heaters to do a built in pool and a shower system similar to yours plus heat the 6000 sq ft home with hot water fan coils. They worked geat but I did rely on a water stack tanks (two 100 gallon tanks) for reserve and a mix down system. Use a 1 and 1/2 inch copper header to tie them all together.It is still in operation today last I heard.
Paloma Tankless Water Heaters by Low Energy Systems
GREAT recovery
171 gallons/hour
100°F rise Per unit
On Demand Gas and Propane Paloma Tankless Water Heaters by Low Energy Systems
These units are very simple to repair also.