I own a Siberian as well and am in the 500 round range myself. No issues at all thus far, like the OP I don't beat the livin" sh*t outta mine and use it like the average shooter likely would. When I bought mine, before I fired a shot, I changed out the trigger for a trigger tech single stage 3lb trigger, and I lock-tited the gas tube and all external screws. My Siberian has been flawless and I can say that I am impressed with it. I am not going to the hills of Afganistan, or the deserts of Iraq, thus I dont feel the need to torture test the gun. I think the gun is worth it, I have no intentions of selling mine.
As for mags, I have only used USGI metal mags and Magpul gen2 polymer mags, both have been flawless with no feeding issues.