So, theoretical maxed out you're looking at 14gpm (which IS a real energy pig, no doubt!).
A 70F rise on 14gpm equals about 490,000BTU/hr.
A single two 80% 500K burner is putting out 400K, so putting two of those monsters in parallel is a wee bit overkill.
If you have 4-8 feet of vertical drain downstream of this font of indulgence, you can get the rest of the heat (and then some from 1 (or two paralleled-up to be able to achieve the flow) drainwater heat exchangers (DWHX) for under a grand per, and the thing would be able to run full blast 24/365 with just one of the hot water heaters. Of the current market offerings, the Renewability PowerPipe series has the lowest pressure drop across the DWHX with flow, and a pair of 4" x 48" or 3" x 60" (or longer, on either) plumbed such that it feeds both the cold side of the shower and the cold input to the water heater should get you there. At 2.5gpm per they'd be returning more than 50% of the heat, and at 7gpm per (14 total) they'd still be delivering over 40%, so with 400K of burner and (0.4x 490K=) ~200K of DWHX you'd have BTU-capacity to spare in showering mode.
It's possible to parallel them up in arbitrarily wide gangs, and getting both the drain an potable flows to split evenly takes a careful bit of plumbing, but if it's even close to evenly-split you'll get there.
Best part about it is that the 200KBTU/hr delivered by the DWHX uses no fuel, and the total energy use is roughly in half when showering, which adds up fast with a gusher like that.
[edited to add] EFI is the US distributor for PowerPipe, and stock a few sizes in their Wisconsin facility. Other sizes can be purchased directly from the Canadian manufacturer, or from that big orange box store if you feel like taking a weekender up north (or ordered online).
The fatter and longer, the higher the BTU-efficiency, but the higher the pressure drop (due to the longer length of potable wrap). Figure out what fits, and figure on at least two. It's easier to balance flows with 4 than with 3, but you can probably get sufficient flow out of 2. Call the manufacturer if you can't figure out how to parallel them, but the above picture of the 4-fer tells the story pretty well.