The Knoxx Recoil Stock Assessment has received mixed reviews (Nutnfancy hated it, others say they love it) here is another review of the stock, the review seems objective and helpful.

One option for addressing recoil that has gotten consistently positive reviews is the Remington Supercell recoil pad, which easily installs on the standard 870 stock. Check it out here:

http://www.remington.com/product-families/accessories/gun-parts-families/supercell-recoil-pad.aspx