A buffer tank will not lower your operating temp by enough to matter (if at all), but it can cure short cycling, if it is indeed short cycling. What do the minimum burn times look like? How much baseboard do you have on your smallest zone?
If your output is 144F and the return water entering the boiler is ~125F and it's firing at min-mod, you'd be getting some decent condensing out of it:
Averaging low-90s with fin tube would be about as good as you can do. With fin-tube baseboard as the heat emitters the output is unreliable and difficult to model when you take the temps below 125-ish an the ODR curves tend not to track there, independently of short-cycling related to it. To get into the ~95% range or higher would require more radiation, and radiation of a type that works more linearly with falling temp. (Cast iron baseboard would work, or low-temp panel radiators, etc.)
The boiler's hysteresis, the output of the smallest zone, and the thermal mass of the water in the smallest zone determines the amount of additional mass you might need. If the minimum burns are longer than 3 minutes, forget about it. If they're under 100 seconds, it's probably worth fixing. Don't know what the hysteresis is on this boiler, but 5F is probably not out of the question. (If it displays the output temp, observe the difference in output temp between when it fires up and when it turns off as it's cycling). At 25K a 3 minute burn would be 1/20 of an hour x 25K= 1250BTUs. A 5F difference with 1250BTU of intput takes 1250/5= 250lbs of water, which is 280/8.43= 30 gallons. But in fact the zone's baseboard is using a fraction of the 1250BTU, and there is likely at least a gallon or two in the plumbing, plus whatever is in the boiler itself, so odds are you'd never need more than 20 gallons of buffer, if the guesstimate on hysteresis is accurate.