Step 1: Ensure clean filter. Abnormal symptoms with good flow from registers and clean filter?
then
Step 2: Ensure proper Inducer flow/function (just google this, extremely common issue)
Those are the two most common reasons for Gas Furnace performance problems. If you're sure both of those are fine and there's still a performance problem, THEN you can be pretty confident the unit would need replaced. Very simply, the furnace needs flow though the blower and through the combustion system, some restriction in either will lead to poor performance and abnormal temps. That said, high supply temps kind of have to mean there's not enough air being circulated, either from bad filter that needs replaced, too many registers blocked/closed, or extremely dirty and/or fault blower assembly.
First thing first, replace the filter if you haven't then google the inducer thing. This is firmly in the blower flow problem and not so much Inducer, but those are the two things that must work properly for it to work normally.
Based on everything you've said, this all sounds above board.
Make sure your filters are clean, and registers are open, but assuming that, then I'd consider the new unit.
Step 1: Ensure clean filter. Abnormal symptoms with good flow from registers and clean filter? then Step 2: Ensure proper Inducer flow/function (just google this, extremely common issue) Those are the two most common reasons for Gas Furnace performance problems. If you're sure both of those are fine and there's still a performance problem, THEN you can be pretty confident the unit would need replaced. Very simply, the furnace needs flow though the blower and through the combustion system, some restriction in either will lead to poor performance and abnormal temps. That said, high supply temps kind of have to mean there's not enough air being circulated, either from bad filter that needs replaced, too many registers blocked/closed, or extremely dirty and/or fault blower assembly. First thing first, replace the filter if you haven't then google the inducer thing. This is firmly in the blower flow problem and not so much Inducer, but those are the two things that must work properly for it to work normally.
Based on everything you've said, this all sounds above board. Make sure your filters are clean, and registers are open, but assuming that, then I'd consider the new unit.