I own/have owned a number a saigas including a number of S12s.
Saigas (and a number of other shotguns) seem to dislike bulk winchester ammo. With the fed bulk mine has run fine from the first shot on gas setting 2.
I was just out shooting some three gun style shooting with my S12 the other day and it reminded me of what a potent weapon it is.
If you are going to shoot three gun (actually I suggest this for anyone who has a S12) I would suggest converting it to a pistol grip configuration. This will give you better balance, ergos, and a much better trigger. Avoid the cheesy pistol grip stock combo from tapco. It offers none of the major advantages of a pistol grip conversion and is a total waste of money IMO. The money is better spent doing it right the first time.
The next best mod one can make (particularly for 3 gun) in my opinion is probably a magazine well. I run a JTE mag well and am well pleased with it. A magwell makes reloads much faster and easier (which means more consistency and no bobbled reloads). Some will say wait for the MD double stacks and a the mag well for them. I saw a lot of that about a year or more ago when I bought my magwell. I am happy I have been enjoying my mag well instead of waiting for a product with no set release date. I also have a drum but I think a magwell is a better setup. An extended mag release goes nicely with this setup.
For versatility and rapid changes a poly choke is a good option. Others have the barrel threaded internally.
One can spend much more on a full out race gun but a very solid performer can be built for around $1000 if you do the work yourself (it is pretty easy do to).
The gun will beat you up a bit.
I must respectfully disagree. I consider mine to be a soft shooters. My S12s are a pleasure to shoot compared to my 20" coach gun or the various pumps I have. After having them for years I haven't even bothered to put a recoil pad on any of them. I am not a recoil junkie either.
Here is a link to a S12 project that I started about a year ago. This gun is not a dedicated race gun but it certainly is up to use in three gun among other things.
http://www.thehighroad.org/showthread.php?t=473230 One day I'll update that thread with the newest tweeks.