Depending on the quality of arm you buy, they might just be good enough "out of the box". When I starting cowboy action shooting, I used the guns I had. At that time, you only needed three, revolver, rifle/carbine, & shotgun. I showed up with my Colt SAA .45, Winch. .30-30, and 1919 Riverside SxS shotgun. They were all "box stock", well except that old side-by-side, it'd been repaired so many times, I don't think anything on the gun was original except the frame and barrels. My only disappointment was that I couldn't use my .30-30. Borrowed a friends '92 until I found and bought one of my own.
My new friends in CAS (now known as the "Wild Bunch") regularly advised everyone that practicing and competition would hone your skills more than action jobs. If you ain't got the muscle memory, no amount of "race gun" is going to make up for it. I still think that spending the money on practice ammo will be of more immediate benefit to your shooting prowess than action jobs, or whiz-bang go-fast parts. When I shot all the time, (2-3 times a month) I generally finished in the top 10% of any match I entered. All the top shooters will tell you, practice, practice, and then practice some more. They may have different opinions on what constitutes "practice", but any practice is better than none.
However, all that said, the foremost reason most new shooters are attracted to this "game" is that it looks like fun! That it appears so is so deceptively simple that few folks actually understand how it came to appear to be so much fun. 1st, and foremost; it appears "fun", 'cause it IS. Each and every contestant determines the level of expectation on any number of levels within the framework of the competition for him/herself. Simply taking time to learn the different aspects of the game in order to determine where you wish to place your competitive emphasis will enable you to better define the goals you set for yourself and make a plan of action to fulfill those goals.
For example: I know a new shooter, who doesn't own any guns to play this game. When asked for my advise on what to buy, here's how I advised him. Meet me at the next match, I'll bring some extra guns so you can get a feel for what several different models feel like. These won't be my "race guns", as they're a little finicky (read touchy). I'll introduce you to some friends that compete in different classes so you can get a grasp of what they're about. Don't worry about buying anything until you've been to a couple of matches, narrowed down the areas that feed your fantasies, then shop away.
That usually snags a lifelong convert to the cowboy way.
Some of these friends have gone on to win major matches while others just plug away at targets, trying for that "clean match" self-award. Some have spent far more time and money on their costumes than their guns.
This is still why I dislike the concept of 2 sixguns for the main matches. It is a deterrent to increasing the ranks of cowboy action shooters. And why I just love the "Working Cowboy" class. Heck, I'd still rather shoot my 10 handgun shots outta the same gun, reloading under the clock. Yea, it's tricky, but... heck even Gene and Hoppy had to do it sometime... even if was "off camera!"

But, I deviate from the original question. Simple answer? Yes, "out of the box" CAS guns are good enough. Some are better than others. You pays yer money and takes yer chances.