You really do not mention the insulation values of the first floor. A 1700 (basement usually has very little heat loss) sq ft home if insulated decent could very well be about 30k. The contractor is measuring combustion efficiency. Combustion efficiency today is normally around 85% to 87%, not a whole lot of change. The thermal efficiency is the big change. How much heat is actually being transferred into the water heating your home. Another big change is GET AWAY FROM BOILERS WITH DOMESTIC HOT WATER COILS IN THEM!!!! Sorry for yelling but maintaining water temperature today is a huge waste of money. Use a three pass boiler (cold start) with an indirect water heater.
Since you mentioned Burnham the MPO-IQ system is unique with the IQ control set-up on it. You can run the IWH right off the boiler and not require a separate relay which is easy to prioritize the IWH. Plug in ODR control and LWCO if needed is also a plug in control.
If going gas the ES2 or Series 3 if you chimney vent. In about 3 months, if on schedule, the IQ will be on a sidewall vented gas boiler boiler which will be power vented or sealed combustion.