Actually, morphine extraction efficiency does increase in heated acidic conditions, however thebaines extraction efficiency increases with heat to a much greater degree. Thus, cold water will preferentially extract morphine, and while the result will have lower total morphine, it will have much lower levels of thebaine, and will result in a cleaner and far more pleasant high.
The heating I was referring to was in regards to post cold extraction, where the resulting solution can be boiled to 1) reduce total volume and 2) open up the phospholipid liposomes to make it faster acting (ie, to increase the rush), assuming that is desired.
Its an unnecessary step however and certainly not called for.