Aside from cost and ammunition consumption, <snip>
What if the Union raised a sharpshooter brigade and gave them repeaters? Would this just be the most elite unstoppable force in warfare in 1865?
The Union did. The U.S.S.S. of Hiram Berdan. The soldiers greatly preferred the combustible cartridge breech loading Sharps rifle (hence "sharpshooter") to the initial issue of the Model 1855 Colt revolving rifle. Nonetheless, the discarded Colt revolving rifle offers a single incident, namely the defense of Snodgrass Hill at Chickamauga by the 21st Ohio Infantry with the five shot Colts that perhaps answers your query about ammunition consumption, and what even a clumsy repeater using combustible cartridges was capable of.
From Wikipedia: "
The Confederates made their first assault on the 21st O.V.I. around 11 a.m. They continued to attack throughout the morning and afternoon, but each attack was repulsed by a murderous fire from the regiment's five-shot
Colt Revolving Rifles. So heavy was the volume of fire that the Confederates were convinced that they were attacking an entire
division, not just a single regiment. At around 3:30 p.m., Lt. Colonel Stoughton, seated on his horse at the rear of the regiment, was fired upon by a rebel sharpshooter. Ignoring the warning, Stoughton dismounted and walked to the front of the line, where another shot rang out and the colonel, struck through the left arm, fell to the ground severely wounded. Command of the 21st O.V.I. passed to Major Arnold McMahan. Stoughton would die on November 19 from an illness that set in during his convalescence.
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By late afternoon, the 21st was desperately low on ammunition. Soldiers plundered the cartridge boxes of the dead and wounded in a frantic attempt to procure Colt's ammunition. When this reserve was depleted, a runner was sent to the rear to search the ordnance trains, but quickly discovered that they had left with the rest of the retreating Union army for Chattanooga. At dusk, having exhausted all of their ammunition, the 21st retired to the rear of the ridge, having expended 43,550 rounds of ammunition.
The 21st was ordered to fix bayonets and occupy the extreme right flank. They managed to procure one last round of ammunition for each man. After each firing their round, the men, remaining in their position, were surrounded and quietly captured. Major Arnold McMahan, 120 soldiers and the colors of the 21st O.V.I. were now in the hands of the enemy.
In six hours of fighting, the 21st Ohio, numbering about 540 men, lost 265 killed, wounded or captured. 46 men would eventually be sent to
Andersonville prison. Only ten of the prisoners would survive."
Past thread:
https://civilwartalk.com/threads/two-battle-reports-of-the-21st-ohio-infantry-at-chickamauga.89881/
NPS Chickamauga:
https://www.nps.gov/chch/learn/news/21stohiolivinghistory.htm
Larry Stevens, Ohio in the Civil War:
http://www.ohiocivilwar.com/cw21.html
Wikipedia on 21st Ohio Infantry with additional links