Practice doesn't matter all that much in terms of judging race pace. The best drivers are now tuning the car for the race, not a qualifying lap in practice. I think that is one of the biggest differences both for Hamilton and Mclaren. If its true that they are heavy on fuel compared to other cars and have struggled with race pace because of that, then it could make an even bigger difference for them. Set them for qualifying then they are too slow with heavy fuel.
Hamilton said after the first race or two that the race pace didn't match qualifying, they needed to set up for a balance between race/quali pace and he's been overall more competitive since then.
Even more important is making tactical choices about setting the car up to work the tyres under the temperature range they think the race will be under. Throw in blocking on track in practice and the times become almost pointless, then you have sandbagging top drivers. Sometimes they are blocked, sometimes you can see them set a purple sector and pull out. The top drivers are more interested in perfecting a specific section of the track, their weakest section, do that but without setting a top time on the whole lap.
However it's still funny that Button keeps pratting on about "I think we've found the fix, all the data from Lewis's good run at Canada, I think we've learned a lot", when every thing including what he and the team are saying is for all intents and purposes saying, we threw out Button's settings and put Lewis's setup on Button's car. Before qualifying and a race there is nothing to see yet anyway.