I almost don't want to answer because I know where you're going with this and I know its a set up; lol!
I think it completely depends upon the kit and the extract/all grain beers themselves. I also think that it would depend upon the style..
I'd like to think that we could tell the difference between a pre-hopped "Can and Kilo" and an all malt extract brew made with steeped grains and fresh hops.
I think most interesting would be which factor makes the most difference? (Is it the use of all malt vs. half sugar, is it pre-hopped vs. freshly hoppped, is it steeped grains vs. no steeped grains?)
I'd love to see:
1. Can and Kilo kit vs. all malt kit (both pre-hopped, same style)
2. All Malt Pre-hopped Kit vs. All Malt freshly hopped kit (same style, ideally)
3. All Malt Kit no steeped grains vs. All Malt Kit steeped grains
Then we could use the metric of how many people could tell the difference between each of the above to determine which factor makes the biggest difference.
I'd be willing to contribute an all malt, freshly hopped, steeped grain hefeweizen to any such experiment if its at least 4 weeks out...