NEW HVAC (Split unit gas heat) A/C not cooling

[kit4u]

I had a new whole HVAC installed over this past winter 2007 (new Furnace(gas), A/C unit, and coil). The house stays cool, but the new A/C (outside unit) runs continously from about 12:30pm to 8pm. It cannot cool the house down below the set temperature of 75 degrees. The old HVAC units 15yrs+ would only run for an 1-2 hours(in August 102F outside) and would cool it down below 75 and shut off. The new unit is the same size unit that was replaced 2 1/2 ton. I called the installer back and he said everything was running ok. He even checked the freon level and the amount of air coming from the ducts (Said all was ok). The air coming out of the vent is about 60-50degrees F. The House has, new windows (low-e, double pane) 19 R attic insulation, and insulated duct work and with walls 10 R.  Single Speed Furnace as the air handler.  I don't think that the A/C should run for more than an Hour or two and cycle off during the hotest part of the day. 

Why should the a/c be running for 8+ hours just to maintain 75-77 degrees?
Why would the much older HVAC system do a better job of cooling the house? (which was on it's last leg and probably leaking freon)
What is the correct volume of air that should be coming out of the duct work?

[Admin]

Take a look on the outside unit rating plate for the actual model #.
Does this ac have a txv installed?
Is it a single stage ac?  Is there a new thermostat installed?
You need 1000 - 1200 cfm of air thru the 2.5 ton indoor coil. (400 cfm per ton)

You'd have to get the tech to write down readings , subcooling, pressures ect...
If you can measure the air leaving the air handler and the air entering the air handler, while the ac is running, let me know what the temperature difference is.