I,v never had a problem with CHT in a light aircraft even when down in the med at temps of +43c or more and I think that the only non-turbo people who do are the glider tugs as they use high power settings at low airspeeds.
The limitation that seems to give me the most problems is oil temp after even a short taxy in a southern european summers day the temp is such that a "step climb" is required to keep the temp out of the red arc.
This seems to be a taxi problem as I came close to shutting the engine down at the hold of 09R at Heathrow on a cool day due to high oil temp after the longest taxi I have ever had.
For me the key to getting the leaning right is the EGT indicator as this gives almost instant indication of the mixture.
As to running at low power setings Lycoming recomend 60% for maximum engine life and I have found that this will give me the best range and would recomend it for the long range cruse but use of this setting all the time has a tendancy to cause the piston rings to stick and oil consumption to increase ,I have found that after about 9 hours of 60% I fly one hour at 75% and this seems to avoid the ring sticking problems.