I had a 2" system with cherry bombs on a 289. I ran a best time of 12.83 with nothing more than a solid cam, 650 Holley and headers. The car ran within a tenth with open headers as it did with the full exhaust.
If the engine was a little lean to start with having the full exhaust on then the gains would not be huge going to open. Did you jet up the carb once you went to open exhaust to take advantage of the better flow? Were you using good long tube headers with 3" collectors? Did you use the appropriate length header collector extensions?
Anything short of a full on rece motor does not need 2.5".
Wow! There are tons of people that make huge gains with street cars going to a 2.5" and even 3" exhaust.
Most of the power gains in a 2.5" system are in your head and not real. Just my 2 cents
You're on a roll. Do you understand that low backpressure is a good thing? Backpressure is definitely good for one thing: lowering power. After the header collectors your exhaust should be as free flowing as possible to make the most power. This does not mean 2" exhaust with crush bends behind a good running V8.