I'm not really in the performace diesel camp but the stock pistons on the OM617 protrude above the surface of the block. If it wasn't for the head gasket the pistons would hit the head. I've put some measurements of a NON TURBO OM617 engine here - some of the data might help give you an idea of the engine.
OM617 (non turbo) cam profile specs, piston height specs etc
You might also want to look through the factory service manual that's available for free via
www.startekinfo.com => but be warned their website is pretty useless...
...I don't think that increasing the compression ratio is going to help produce more power in an OM617 though - work in other places would probably be beneficial.