Ballistic Products publish a booklet called The Mighty Ten Gauge that has various revisions that update it. I recall seeing a 2,1/2 oz recipe in one of the older ones. Keep in mind that, these overloads usually don't put more pellets in a center circle than, say, a 2oz load because the wad column has to be shortened to allow the extra shot. That means it has less cushioning, the shot sustain more recoil damage and hence they pattern less densely. You can instead use buffer and/or copper or better still nickel plated pellets to up the density to get, for arguments sake, more hits on a target like a turkey's head. I've not heard of a 3oz load except in an 8 gauge.
I have used heavy loads in old doubles, but I usually drop the charge a few grains. It's still not good practice and not something to do very often, for the gun's sake or for yours. If you end up with a flinch, you'll have a 2,1/2 oz miss instead of a 2 ounce hit.
I'd suggest doing some patterning until you have a good load you can trust. Try using a Ballistic Products BPD wad rather than a SP10 Remington one. You can alter the slit of a BPD (Ballistic Pattern Driver), wad to get denser patterns. You'll also see how your 2,1/2 oz compares on paper. Draw a 20" circle around the densest part and count the hits.
