Some historical notes
According to Osprey book on the subject Union raised 11 regiment size units, CSA raised 16.
Recruting was very selective and training was much longer than regular infantry. Equipment and supply was of above average quality.
To sum up, elite units. IMO that is not accurately protrayed in the game. Sharpshooter model has indeed a longer building period than regular infantry (40 to 60) but the cost in war material and money is pretty much the same. IMO that is not realistic because players can easily afford to buy them.
In reality Sharpshooter units fought in groups of 2 companies, not entire regiments. (that would provide 55 units to Union and 80 to CSA).
My suggestion would be to have a really slow building rate (20 or 25), and double money and war supplies spending. Also to restrict the number of units available, or otherwise to make units 2 comapay based.