Were the iPads you bought 4-6 months after release NEW units or refurbs? Buying a refurb 4-6 months after release is like buying a new one on day 1.But is there any proof that things do get better after 4-6 months? In my experience, this isn't the case but that's my opinion
Since Apple doesn't offer any manufacturing reports, there is no "proof" other than anecdotal evidence. Of all the iPads that I've purchased since day-1 of gen-1, I have yet to have one with backlight bleeding, dust under the screen, uneven colors, etc. All but the first (pre-ordered) were bought 4-6 months after they were released.
That's only about a dozen iPads or so and clearly statistically insignificant. But it is my first-hand experience.
I haven't seen anyone posting complaints about their newly purchased iPad screens 6 months after release, and certainly not returning 3, 4, 5 or more times to get a good screen.