It all depends on the valve you plan on using. If you aren't using a specifically built valve for high pressure - e.g. unregulated co2 @ 850psi - you should use a reg someone. Check out PPS's HP LT valve, with one of those you can run the 12g changer directly into the VA, where your inline reg would normally go. If you are using a stock valve, or common low pressure/mid pressure autococker valve (these are the majority of aftermarket and stock valves out there), you will get the best performance by using a reg. You have al ot of options here, using the reg mounted in the VA like stock setups or running a female stabilizer or some PPS specially mounted reg to accommodate the 12g changer.
So if you want to run a true stock class setup on your sniper/autococker pump, you need to look into high pressure (assuming you don't wanna deal with regs, which is my suggestion) as well as a spring kit. In any case, no matter what you do, a spring kit is pretty much necessary to get your gun tuned properly to run off of 12gs.
Good luck! Let us know what you end up doing. I personally love my old pre-97 r/f sniper, which uses a HP valve, maddman spring kit, and runs off 12g in the VA where the inline reg would normally go. Get about 30 shots per 12g, give or take. Great gun, lots of fun, nice and beefy design that ends up balancing perfectly to my tastes.