Lets start by assuming that it works pretty well as it is.  If you are not wildly overspeed and the overhead line tension is OK, then what you get by adding head shims is less torque and a softer break, so that the speed increase when the shift to 2 cycle occurs is mild.  More compression makes it pull and break harder, and in an extreme case, can make the airplane surge or forge ahead in corners and when brought to vertical upward flight.  Less compression softens the break, and in an extreme case the engine may make less torque when it is 2 stroking.  My guess is that this is because of the higher oil content when 4 stroking raising the effective compression, and the better scavenging while 2 stroking sweeps out the excess oil.
  Lower compression won't fix an R/C engine which wants to run away, the liner timing must be modified to stop the runaway.  The most important part of all of this is that changing compression is only one of about 20 variables which all have to be optimized.  Before getting to compression, try to get at least the line length and prop/fuel/plug choices done.  You should be able to match the prop to the available torque to get the level lap speed about right.  After that, you can start to evaluate the break, and decide whether you want more or less 'kick' when it starts to 2 stroke.  A good way to deal with an engine which is not overspeed (too much torque) but has too much break is to hemi out the combustion chamber.  You lose a little compression, but the hemi, even if all you do is to round the edge of the squish band.  The hemi chamber shape or at least one without a wide flat squish band has always given me a smoothness to the break that can't be found any other way.  So you can play with deck height, chamber shape, and then try another round of prop changes, using a little more blade area or pitch to give a little more bite.  Hope this admittedly incomplete bit is some help.  It is a long term learning process.  Tom H.
 

If its a stock LA 46, the break should be close in stock form.  Try ignoring the ground run, and make changes in the setting based only on how it ran in the last flight.  Steady 4 cycle, needle in 4 clicks and fly it again.  Steady 2 cycle, back the needle out 4 clicks and try it.  Especially if it has the dreaded rear needle valve, but to some extent with any uniflow setup, the ground run is not a perfect prediction of the in flight setting.  Tom H.