First of all, there's really 2 types of '97s on the market: the Winchester made, and the Norinco made. A lot of people love to shoot the real thing. I for one have an original '97 in the stable as well as the Norinco version. Both are reliable and both deliver the shot down range. If you really want an Winchester, they can be found but supply is starting to get diminished since they quit making them in the 50's and the ones found at gun shows can range from complete basket cases with bore pitting, dinged stocks and rusted barrels to absolute black-diamond variant cherries. It's all of your preference. Any '97 you buy should be checked by a competent gunsmith before doing any CAS matches with it.
If ya do want a winchester, you can find two flavors- the takedown or a solid frame. Either are sufficient for CAS and neither has an advantage over the other, IMHO. If you do find a takedown and you intend upon using it for CAS, I'd suggest getting a gunsmith to really tighten it down and never take it apart. I think the gun integrity is better if none of the takedown parts move or can move during a match, plus the takedown collars are in rare supply if ya ever dork one up.
Early winchesters (D and E versions, solid or takedown) had the forcing cone really short. I don't have the specifics of dates, but early winchesters were originally bored to accept 2 5/8" shells and the chambers were very blunt shouldered forcing cones without any angle or slope to the forcing cone section. Why the fuss over the forcing cone? A properly lengthened chamber & smooth forcing cone will provide less recoil and better shot patterns. Besides measuring the forcing cone dimensions, look at your expended shells. If they are still in the "crimped" position as opposed to being completely opened up, you probably have a forcing cone issue.
Ironically, the Norinco '97s (sometimes called Norchesters by the cowboy-types) suffer the same fate. Apparently the original '97 dimensions were copied from an early D or E frame and subsequently the forcing cones are squared and short. I picked up a 3rd gen Norinco a short time ago and had Squibber do his magic action job on it. As expected, the chamber was short, the forcing cone was a blunt shoulder.
A buttpad or recoil pad is useful for two reasons: it reduces felt recoil, and it helps anchor the shotgun into the shoulder so repeated shots don't have your adjusting your gun position. A good stock length is crucial to accurate and quick shooting. Any trap, skeet, or sporting clays shooter that is serious will have the length and comb of the stock adjusted to his/her preference. Same goes for CAS. I've found a 2" shorter stock works better for me. I've found that as it eliminates the gorilla-like reach on the pump arm and having the receiver groover closer to my eye forces me to sight in the front bead better.
Since we're talking about sights, it's a good segue into the front sight. Original Winchesters and Norincos all seem to come with dinky front beads. I had mine replaced to a 3/32-sized brass bead. It's big, it stands out, and it makes a point of where I need to aim. Bigger is better in this case.
Finally, pattern sizes does count. My guns are choked to improved cylinder, but they have removable chokes for those rare occasions that I go dove hunting with it or participate in cowboy clays. I have my barrels cut down to 20" so having a choke is almost required for decent patterns and good knockdown potential.
Action jobs are almost required for Norincos. An action job smoothens out the rough edges in the receiver, cleans up the metal on the carrier, and basically makes it a smoother operating gun. Norincos seem to have the trigger position to the very rear of the trigger guard. One of the gunsmiths I know re-heats and bends the trigger assembly to bring it more forward so it feels like every other gun and eliminates the trigger creep.
So who's the best for action jobs? It's all opinions. Personally, I had Squibber do the action jobs on my '97s. He's darn knowledgeable on what to do on a '97, what pieces need tuning. I had him do an action job on the Norinco I purchased: shorten the stock, add a recoil pad, do an action job to make it smoother, cut down the barrel, install a choke system, replace the front bead, and pin the front handguard down so it will never strip out of the collar. In his action job he not only made the gun really smooth, but he brought the trigger forward. There are other gunsmiths out there who do quality work, but I've always had great work done by Squibber so there was no need for me to go anywhere else.