Compression ratio

I'm looking to get a little more power out of my isx Cummins. It is currently set at 500hp and 1850 on the torque. It was made in 9/2011. Cummins told me I was at the top of my current cpl. I know a shop that can do a custom tune for my truck but said they don't want to make it unreliable "I don't want that either". They told me to find out what compression ratio the pistons were in the engine. The truck had a new engine put in before I bought the truck and I did some digging and found that the piston kits that were put in are a part number 2881758. I have talked to Cummins and peterbilt and no one can tell me the compression ratio of my piston. I was wondering if anyone can help me out?