Although I have asked several questions on "would have + past participle" construction, I still have a few loose ends to tie up.
As Leo adequately pointed out, my doubt is when to use 'simple past tense / present perfect tense' and when to use 'would have + past participle'. (Let's dismiss the irrealis usage of "would have done" in this question)
Consider the example he contributed in his answer to my previous question:
Let's suppose you are a policeman investigating a burglary in the rich neighboorhood and one of the suspects is the neighbour who happens to be rich and you found his fingerprints inside the house where burglary took place.
A jounalist might come up and ask you,"Why would he have stolen the money?", or he could ask,"Why did he steal the money?"
A: Why should I believe you? So I can end up in here with you?
B: It's legal. Ask the IRS. They'll say the same thing.
A: I feel stupid telling you this. I'm sure you would have investigated. (Why not say "I'm sure you have investigated"?)
B: I don't need you to tell me where the bear shit in the buckwheat.
A: Of course not. But you do need someone to set it up for you.
-- The Shawshank Redemption (1994)