Will a 90+ furnace use less fuel then an 80 YES.
But the return on investment needs to be looked at in 2 ways.
1 Your monthly expenses.
2 Your utility supplier.
The 3, 5 or 10% you save as a consumer is helpful but in the big picture if we don’t begin using less electricity and fossil fuels more power plants will have to be built. More pipelines will have to be laid. More wells dug.
This whole argument over 80% / 90+% is not about one house but your local utility company’s ability to continue supplying the consumer.
Why do you think A/C’s have to be 13%SEER.
There is a proposal to put a LNG storage ship in Long Island sound the size of the Queen Mary. Would you want this in your back yard. I don’t think so!
Think about it.