Ok, so let me get this straight.....the property is a two level home, the lower level seems to be fine with the heat and cooling, the upper level has plenty of air moving in the ducts but there is a discrepancy of 10-15 degrees between the AC unit and the end run of the ducts as they enter the rooms. One room has a much reduced flow of air to it.
Have you, yourself, put a thermometer at several ducts where they enter the home?....this would help to find out if it's just one or two duct runs, upper and or lower level only. If you find out that it's only in the attic run ducts then the attic heat would be a real issue. (Attics should never be allowed to reach 130 degrees regardless, it for one dramatically shortens shingle life).
Another approach to take would be to place a thermometer under the attic duct insulation, find out what temp. it is reaching there.
Cleaning of the ducts will change the temp. very little if any, what cleaning does (other than reducing particulate matter and all the other pollens etc...) is help keep air flow up.
Changing to a bigger unit doesn't mean that you get colder air, a bigger unit is able to pump larger volumes of air to cover larger areas. As a rough guide for approximately every 600 sq. feet you need 1 ton....this is a very rough guide as there is a whole system of figures to working out exact sizes, but it gives you a good ballpark.
From experience I've found that in many cases when a tenant see's a landlord having problems with a particular repair sometimes the line between "it's never done that".....to "oh.....it's done that the whole time lately"......becomes blurred. That's why I think you need to work on an elimination approach, if you actually find that there is a big drop in temps. in all rooms upper only then it's an attic insulation/airflow issue.
Regardless of this issue you would be very well advised to have a powered roof vent installed, ideally you should be able to keep your attic temps. down around 90 degrees in mid summer, remember though if your sucking air ou of the attic you need to make sure that the soffits can allow fresh air in.
The foloowing link is a great explaination of roof venting and while it is from Texas A&M University, the same principle applies anywhere.
Attic Ventilation
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