98% efficiency is much less a function of the flow rate & output temp than of the incoming water temp. The output temp is only a 3rd order factor, but at low flow low-fire the lower turbulence on both the fire & water sides of the heat exchanger can start to create insulating laminar flows that'll knock a few points off the raw combustion efficiency. Whether it's actually 95% vs. 98% at minimum fire is hardly as relevant as what it is at 20-40% of full fire from a total fuel use point of view. But whether the output is 100F or 130F makes less than 0.01% difference. But if the INCOMING water temp is much over 90F you won't be making 98% thermal effciency at any output temp.
Sorry to hear that their customer support is so abyssmal though.