I would suggest 就像真的 is the closest translation. First, it is literally close to "as if". Second, it also has the implication of disbelief.
In your example:
A: 过两年我就开法拉利了。
B: 说得像真的一样 (means 别做梦了)。
"废话!"literally means "redundant utterances". Its equivalent might be "You bet![with an unpleasant tone]" which implies you actually agree with him.That's diametrically opposite.
