If your space was designed for a 1.5 ton system, i find it hard to believe that you would consider a 5 ton. The furnace and coil to match if you found them would blow your windows out...way too much airflow. Can you say mold, mildew from short cycling and no humidity removal, energy consumption would be crazy, i could go on but this is a bad idea no matter how you look at it. The server room had a high heat load from servers and needed that size, i seriously doubt your standard residential home has that size load.
The 5 ton system you have is not 500 hp racecar engines they are 500 hp truck engines. The duct work is the transmission if you 3x the engines hp without upgrading the transmission you will have problems.
The undersized ducts will not move enough air and liquid refrigerant will likely get into the compressor and damage it.
Most air conditions have only 2 operation conditions 100% on or 100% off. Now lets imagin you want to drive your truck engine equipped Camry in stop and go traffic and your only speed control is engine 100 % or costing. Your engine will get lots of very short run cycles.
Short run cycles are bad in 2 ways 1 it is hard on the equipment 2 AC equipment does not start removing humidity from the air until it has run for over 15 minutes. Lots of people are comfortable 78° at 50% humidly. With short cycles you are more likely to end up at 78° at 80% and uncomfortable so the set point gets turned down and 68° at 70% is bearable most find it cold and clammy.
Also it can be bad for your health as the air in your ducts will likely be 25° below the set point. 43° ducts exposed to outdoor air will be well below the dew point water will pour off the duct and mold is possible
Do you think the 500 HP Camry gets better MPG?
Its more about run time with high RH removing, than to large over sized AC unit, in houses. The job I am working on now is started with 9 tons and will bring it down to 5 ton need. Now the duct size is right with just a total of of .5 SP just as the air handler is listed. Most jobs I do have very large loads to start with and when I finish its much smaller load. I see very high pressers and low CFM then the coil freezes on bad duct work jobs. I have seen 400 SF IT rooms take 20 tons of cooling and then change hard drives to more energy lowering now 9 tons and more speed. Most IT rooms are high CFM and low RH removal.