I have a single shot full choke 410 that I would like to try and hunt quail with.
Ive pattened Remington 3 inch 7.5 shot at 20 yards and wasn't impressed. The pattern had a very dense core about 8-10 inches in diameter, and sparse fringes full of flyers around the entire center.
I assume this is because of the large amount of deformed shot due to the full choke?
Would 2.5 inch shells pattern any better?
First, "better" needs to be defined. In this case, you're looking for more open.
The obvious fliers relate to the long - and only partially protected - shot column. Your real problem is the full constriction. 2.5" shells will dump a higher percentage of their payload into the core, IME. Core will look less dense to your eye b/c the absolute number of pellets will decrease. But that might, at least, help you not burgerize the quail.
Unless the gun is collectible I would open the constriction to about .005. You can also try some different brands of 3" shells. Those RPs are my favorites for wingshooting, but I'm using around .008. I found the WW 3" shell throws a slightly more open pattern than the RP. The Fiocchi 3" load (at least, the one I dissected) uses a wad that lacks petals. That might open the pattern to suit you.