At least one author on usage (Wilson Follett, Modern American Usage, 1st Ed.) maintains that a sentence like "He behaves as if he had done it all himself" is a reduced version of "He behaves as he would behave if he had done it all himself."
In other words, if he had done it all himself, he would behave in a certain way, namely, as he does. On that basis Follett proscribes the present tense, including the present perfect, in the "as if"-clause, and prescribes the subjunctive where applicable:
"He behaves as if he were out of his mind."
(He behaves as he would behave if he were out of his mind.)
I generally shift to "as though" when I wish to use the present tense in the clause in question: "He behaves as though he has done it all himself"; "He behaves as though he's out of his mind." Follett does, however, point out that "though" means "if" in that construction!
Informally, of course, there is always the "like" version.