Are you sure 4 gpm at your main incomming, mine varies on time of day
and season.

70 f rise , did you test water temp at-near the heater and the loss it
experiances at its use point, you should measure shower head output
temp, I loose apx 5-10f.

What is your temp incomming lowest, in a cold year, test now but lower
it 10f easily.

Cross comparing can only be done possibly on a specification since
rebadging, recasing, or a single line for Bosch is likely.

It is better to upsize than make an expensive mistake. Although I do
fine with 117000 btu and 4 gpm and 33f incomming not even set to high,
my shower output might be 1-2 gpm. Be sure your gas can handle the extra
load, im sure your furnace is not near 180000+ btu. Load must be
measured with all apliances on.

Look for a unit that maintains output temp and has a remote thermostat,
my unit does not maintain output temp, it maintains temp rise, so in
winter I must turn it up manualy. But I am happy with the savings I get.