Based entirely on the pilots i have flown with there is a degree of complacency that sets in after the first 100 hours running up to the first 600 hours. Beyond 600 hours it would seem they have usually had a fright or two, which is inevitable; to still be flying after 600 hours also means they probably take their flying more seriously.
I have flown with some pilots in the former category that i doubt would pass their ppl.
Were there a properly controlled study which adjusts for the factors raised earlier i would not be surprised if there were some truth in the premise.