The opposite is true- you get higher combustion efficiency (and higher system efficiency) at LOWER temperatures, but the critical temp is the that of the return water, not the output water. See:
But for cast-iron boilers you have to take precautions that it operates primarily ABOVE the temperature at which the flue gases condense. For natural gas boilers you're generally safe it the return water stays above 130F. You can sometimes go somewhat below that, but at a risk of condensation in the flue. If you have a stainless flue liner and something to catch & dispose of the condensate you can take it right to the limit, but don't let it condense on the boiler's heat exchangers or you'll shorten it's service life considerably. The condensate is slightly acidic- similar acidity to red wine, and can be dumped in normal drains.
At 120F you'll be getting condensation on the plates, but if it spends most of the burn time above 125F it's not a disaster. Setting the high-limit at 145F seems on the low side though- since the return water from the radiators/baseboards is likely to be 20F colder and in the condensing zone. But if the boiler is plumbed with a "boiler bypass" pipe mixing some of the output in with the water returning from the radiators, or if it's plumbed with dual-circulators in a primary/secondary setup, the water entering the boiler might still be sufficiently hot to protect it:
It sounds like somebody cranked it down to max out the efficiency already- hopefully they didn't go too far. But if you can carefully measure the temperature at the point where the water enters the boiler you can verify that.
How long do the burns take to go from low-limit to high limit? If it's under 5 minutes you're probably losing much of what you've gained in lower temperatures to cycling losses, but if it's over 10 your good. If it's really short, raising the high limit to lengthen the burns may be beneficial- I wouldn't take the low limit any lower- it may be too low already. If the boiler is just plain too big for the heating load (which is often the case), you can replace the aquastat (aka "the gizmo") with a smart controller like the Intellicon HW which will raise/lower the high limit based on how fast the water temps are changing to maximize efficiency. See: http://www.intellidynellc.com/02_pgHW.htm There are other smart boiler controls from other vendors, but this one is an easier install for the DIYer, and it does a decent job of it.