I haven't heard about changing shims to alter the amount of power you get, unlike a petrol/gas engine, you retard the timing (inject fuel sooner), you get a little more bang for your buck, but comes at a price of potential engine damage due to engine knock.
After a quick search, retarding the timing a little does seem to give more power, and does have the same potential for engine knock as a petrol engine has.
Personally, i'd leave it alone, they were tuned at the factory to run all day long without issues, if you mess with these adjustments, you do run the risk of shortening it's life span. Without throwing say a turbo on it, i'd like to think the gains you'd get with messing with adjustments, would be very minimal.
These engines only produce about 15 horse power, you may get a tiny bit more out of messing with settings, to me, it's not worth it. and it will be a very low number.