The difference between “as if” and “as though”

There's no real difference in contexts like "It was as though a great weight had been lifted from my mind" or "It was as if a great weight... etc."

However, the sarastic use of "As if!" favored by teens and adolescents (which means "That's not likely to ever happen") just doesn't work with "As though!"

There is no difference between "as though" and "as if". They mean the same thing. You can say both of these sentences:
"He acts as though he were the boss."
"He acts as if he were the boss."
He looks as if he has finished the test.
(perhaps he has finished.)
He looks as though he was leaving.
(perhaps he was leaving.)