I am simply going by my own experiences, since all my plants are potted. Growing mature plants in pots is different than growing them in the ground. This may sound counter-intuitive.
Even though this plant has grown oh-so-slowly, it is still more than 4 yrs. old. Perhaps it needs more potting soil to grow a more vigorous root system. Even though it has probably been hybridized to death, it is still a tree (genetically). Why not up-pot it into a 5 gal. pot (minimal) and see if that helps. Having more of a expansive root-ball should also help give you better moisture control (I try to keep all my plants pretty much constantly moist spring through summer). I water every day but since I make my own potting mixes, I know my plumeria potting soil is very well draining. Having this constant moisture to draw from, my plants seem to handle the harsh sun better. So far I haven't seen any sunburning.......knock on wood.
