Proof? Proof is in the hundreds of posts on this forum, on Xtremesystems.org, on Hardforums, and on dozens of other forums where many many people have purchased OCZ and Corsair and found mixed results.
They basically always do what they're rated for, but outside of that, it really is hit-or-miss. Nobody has been able to show that their OCZ or Corsair will overclock by any consistant amount. Some will go right up to their rating and not do 5mhz over; others will go right past their rating to speeds of 30+mhz or faster.
For the moment, everyone who has puchased the Komusa PC4200 ADATA parts has consistantly found they perform at PC4500 or better unless limited by CPU or motherboard. That doesn't mean they'll do 300 at 1:1 on the first set, but it's likely you could find a set that will do 295 1:1 without too much trouble.