Is my existing ducting undersized?

I have the ceiling down in my kitchen (the closest room to the utility closet where the furnace is). I have a 2 ton Bryant 925t furnace, which according to this page (Gas Furnaces Matrix) puts out between 1000 and 2200 cfm. On both the supply side and return side I have 14x8 sheet metal ducts. The supply side eventually goes down to 8x12 after 15 feet or so and then down to 8x10 further on. From what I understand, 14x8 can carry 500 cfm (http://www.northwestenergystar.com/s...Deck_Final.pdf) Should I replace the 14x8 while the ceiling is down and if I only do that section (and not after it goes down to 12x8, etc) will that really make any difference?

Thank you for any advice

 

14x8 is good for 550cfm supply 450 cfm return

 

To give a more direct answer, yes, your ductwork on both the supply and the return is woefully inadequate.