What is the cost of one percent efficiency on an oil heater?
Try this on for size:

Annual Oil Bill * Efficiency = Cost of Oil actually used for heating. (The rest is waste).

So on an annual bill of $1000, you are currently paying $650 for heat, and $350 out the chimney. (i.e. $650 is as low as you can go with a 100% furnace)

So, if you were to replace that with an 87.2% furnace, your annual cost would be $650/87.1% = $747, and there is about $100 waste. (Note! this is a savings of $250 per $1000 of Oil)

On the 86.1%, that's $650/86.1% = $755 for a difference of $8 per thousand of oil currently used.   At that rate, the new furnace would take 375 * (time in which you use $1000 of oil) to pay off the difference.

This analysis is based purely on BTUs produced at the burner.  

Now, there are other factors, such as air flow and distribution which may significantly affect your oil use, and the bigger furnace may be more efficient in other ways.