Adding an extension method ( say AddCollection) is a bad idea since it accepts an argument of IEnumerable(T).

using items.Count() is slow - I guess it uses a foreach loop to count

// don't do this at home

public static void AddCollection<T>(this List<T> list, IEnumerable<T> items)
		{

			if (items.Count() <= Tuning.AddRangeMinimumItems) // 128
			{
				foreach (T item in items)
					list.Add(item);
			}
			else
			{
				list.AddRange(items);
			}
		}

An extension method that takes a whole array is a better idea - since array.Length is faster