Brand-new 2-ton not cooling 1200sqft enough in Austin

by Ian M.
(Austin, TX)

Last year I was excited to upgrade my energy efficiency with insulation and a new central air conditioner, to stay COLD in the HOT summers, and pay minimally for the electric bills!
Replaced a central air conditioner 11 years old, it is as old as the house, Ex.BTU's 24, SEER 12, while new has only 20,200 BTU's but higher efficiency.

The central air units has worked great, even with Aprilaire HEPA filter changed every 6 months!
However, last weekend, was 100deg.outside, "cooling" to 90. I leave ac unit on at 78 all day, but switch to 72 when I get home. Last week it took till middle of night to get to 72. 2 weeks ago I watched as the temp rose to 76 with the central air conditioner on!

My Ac guy says air conditioner systems can really only comfortably cool to 20 degrees under the outside ambient temps.

My central air conditioning units is: 2 ton American Standard R-410A 16SEER/13 EER with Aprilaire filter provision. I also upgraded attic insulation to 38 R-value, and had a big gravity vent added.

So, I expected this air conditioning to cool on demand, and not have to either wait 8 hours for it to really cool, or leave it at 72 all the time.

The ac unit has actually worked flawlessly for what it does, but:
Question is, does this sound like an air conditioner problem? Or a house-losing-all-the-cool insulation/window and too dang hot outside problem?