Thee is a body of opinion among some, and I tend to agree, that had Forrest gotten command in the Western theater after A.S. Johnston’s death, the Battle of Shiloh, and maybe the whole western theater would have had a totally different outcome. But both sides had their share of idiots in command.
But if he had gotten the command after Joe Johnson, or if Stephen Lee hadn't kept him in Mississippi when Forrest went out to cut Sherman's supply lines, things might well have been different.