They sell thinner materials for the backsplash (3/8"). Grinding it done is un-doable and absolutely crazy. Removing the beadboard if needed will be much much much easier. If the granite sticks out more than the rest of the beadboard this will still look fine. Removing the beadboard is not very hard at all. Maybe a 10 minute job at best. Borrow a multi-master tool to make a perfect cut where the granite will go and the rest of the beadboard will stay. If you cannot find one use a sharp wood chisel. Pros can do it with a sawsall, but not the best tool to use here for a DIY job. Also an angle grinder with the tile bit will make a perfect cut and then finish the edges with the chisel.