Steady state combustion efficiency is one thing, the as-used AFUE is another. Most homes in CO will have a design heat load under 40,000BTU/hr which means this pig is probably 3-4x or more oversized for the actual load. With standby & ignition cycle losses running maybe 65-70% true as-used efficiency. A right-sized cast iron boiler would pretty much hit it's AFUE numbers (which presume no more than 1.7x oversizing), and a right-sized modulating condensing boiler could even beat it's rated AFUE if you have enough radiation and a system designer who knows what they're doing.
If you are heating your potable hot water with an embedded tankless coil you have HUGE standby losses. Buying an indirect-fired hot water heater (or a stand-alone hot water heater) and setting the boiler up to run much cooler plus utilizing heat-purge controllers (eg Intellicon 3250HW+) would likely pay for itself in under 12 years, provided the beast lives that long. If you're burning 1500 or more therms/year a ~$150-200 DIY-installed heat purge controller will pay off in under 2 years, probably less than 1 year, depending on the oversizing multiplier.
The better thing to do is make a replacement plan and execute on it. If you have a fuel-use history (and aren't heating half the place with a wood-stove or something) you can size the replacement boiler reasonably without running a formal Manual-J by using fuel-use against heating-degree-day data. With a zip code (to look up heating degree day data from a nearby weather station at degreedays.net and to come up with a 99% outside design temp) and the fuel use between exact billing dates, using the name-plate efficiency of the boiler it takes maybe 10 minutes to come up with a number close enough to use for boiler sizing. If you are planning major upgrades to the house that involve more air-sealing & insulation it's worth running the "after" picture calculations if you're not already at the smallest-of-the-line for boiler sizing, which is where MOST houses should be.
