I'm supporting both you guys lol.
I think your case plays more into the 10 lap averages not being an indicator. Cars the post lap times will be fast, but that doesnt mean the car will race good. In contrast, that doesnt mean the cars wont be able to be adjusted.
Suarez has a very fast car this weekend, but a combination of inexperience, changing track conditions, and other teams adjusting their cars better, the 19 likely wont be able to maximize their speed with a car that is handling to Suarez' liking that can compete for a win. I think this is the point you are making, and I agree.
However, final practice provides an indication on which cars were *fast*. That itself I dont think can be denied, because it can be an indication of how the race may be..it also depends on the team, and their history. The 4 for example..if they are fast in practice, they are fast in a race, regardless of changing weather conditions. Carl Edwards used to be like that. Brad was...KB not so much..from 2005, practice wasnt always an indication on how he ran, for better or for worse. So the 18 team plays into what you are saying, both with winning cars and struggling cars. But some teams who are good in practice usually are good in races, regardless of the weather.
So you guys are ultimately both right and wrong. It's definitely not black and white.