I don't get why you'd want or need 140F in combination with recirculation under any circumstances, but it's pretty much guaranteeing the highest possible parasitic heat losses. And if it's less than 100' each way to the remote tap the recirculation burns are all short-cycles too. It'll only get to 95% efficiency is when
A: The water entering the heater is under 100F, and
B: You're pumping more than 5-6 gallons through it per burn at somewhere around 1/4-1/3 of full-fire.
Short-cycling it with 2 gallon burns @ 130F return water it's probably getting 50-60% net efficiency at the heater, before distribution losses.
Then, considering you probably didn't insulate the plumbing runs to more than R2 (if at all) on the distribution/recirculation path your heat loss from that path is HUGE. I don't even run the tubing on my radiant heating over 130F- EVER, and it's doing a pretty good job of keeping the place warm. Your recirculation plumbing is just a long skinny heating radiator.
If you plunked down a tank in the same spot, at the same temperature, with the same recirculation scheme, you won't save more than a dime or two in operational cost, and it may be a dime or two the other direction. With a tank heater you get an ~80% efficient burner but standby losses that take it way down, and if you're dumping all that standby heat to the garage, so you don't even get the heating-season benefit. You'd still have the huge distribution loss of all of that hot pipe hanging out there.
Crank the temp down to whatever the hottest you'd need (probably 115-120F, to fill a big tub.) Run the recirculation on a demand basis, not continuous, and insulate the plumbing to at least R4 (5/8" wall closed cell foam), but R6 or more is better. (Some plumbing supply houses may have 3/4-1" wall closed cell foam pipe insulation, box stores won't. Grainger carries some selection too.)
Also, 2 Naviens? Do you need to fill a big spa at 8gpm at the same time that 3 showers are going elsewhere in the house or something? (That's a heluva lot of burner there!)