Wish I knew. I'm just planning on providing overkill so I don't have to worry about it. I'd guess wash temp would be closer to 95-100 F.
OK, so 5 GPM with half the water supplied by a heater set at 160 F would give you a ~30 minute shower.
Two 80 gal tanks would give you one hour.
Or, if you can get heater elements that put ~35 kw into your 80 gal tank you have in effect made yourself a 5 GPM 100 F "tankless" heater.
The problem now is to find a 35 kw, 146 A, 1.6 ohm heating element that is mechanically compatible with your existing heater. You probably won't find one; it would get so hot the water would boil locally and impede heat xfer.
Another way, using any 300 gal. tank holding 2500# of water.
It takes 1.3 therms, 37 kwh, to heat this 300 gal by 50 F.
At 4500w from your regular WH it takes 8 hrs @ 0.6 GPM to fill the tank with hot enough water.
To avoid math errors and do tradeoffs put this on a spreadsheet.