On the Second Day of the Battle of Gettysburg, the Third Confederate Brigade under Brig. Gen. Ambrose Wright (serving under Anderson's Division) charged from Seminary Ridge into Union forces holding Cemetery Ridge, overrunning two batteries of Union guns and nearly capturing Meade's headquarters. My great great grandfather was a lieutenant in the 48th Georgia Infantry which fought on Wright's left flank. To their left was Brig. Carnot Posey's brigade of Mississippians who, except for elements of a few companies, did not advance onto the ridge to protect Wright's left flank. The Georgian's hollared, "Come on you Mississippians," but they wouldn't budge because Posey had given no orders to. After the Battle, Anderson took the blame for not giving Posey adequate discretion, but the question is, what would have happened if Posey had joined the battle? Reportedly, Wright's forces went further than Pickett's "high water mark" on Day 3 of the battle, but was forced to retreat when the 48th and other units took heavy casualties (my ancestor was wounded). Have any military experts studied this to see whether Posey's forced might have won the entire battle for the South on Day 2?
Posey's brigade had only 1300 men. Wright was advancing into the gap left when the III corps under Sickles advanced out to the Peach Orchard and Emmitsburg Road. Odds are that Posey would have been advancing into the II, I, and XI corps units on the hill that the next day would repel Pickett's Charge. Posey would have been exposed to flanking fire from the batteries and might not have gotten as far as Wright, who had some 'cover' due to his breaking the forward lines and having Union troops in the way.
Another consideration is that VI corps had arrived and was being deployed to plug the gap, and units from XII corps were also rushing to the area. The Union was about to get very strong in that area, which is why Wright retired. Posey's men would not likely change the balance all that much.