I use the slides as a baseline so I can compare actual results when independent reviews come out. I admit though I don't keep track of all the nuances, cataloging claimed vs. actual data, don't care enough. But from what I saw with Broadwell's Gen 8 GPU being claimed at 25-50%, from Gen 7.5 HD 4400 to Gen 8 HD 5500 there were some benchmark increases within that range, and some a little below + some higher.
Obviously there will always be marketing spin, but it's doubtful that a company is going to egregiously lie in their slides. There has to be some acceptance that the actual results won't deviate too much. Their goal is to be as trustworthy as possible (while also portraying themselves as making significant improvements), so I think you might be going a bit far by not putting any trust in them.