LSAT prep companies and book writers do not, to my knowledge, have access to any special information the general public doesn't that would answer this questioj. LSAC obviously would but they also have an interest in people believing that if they just keep writing they can improve.
The only information I've seen on this is what Cleanhands linked several pages back which doesn't seem favourable to their business model which I'm willing to infer is probably why a business with ~$70,000,000 in annual revenuebstopped releasing it.
Besides, the person disagreeing with me initially was disagreeing with the proposition that people top out in terms of improvement eventually; you just need to buy that not everyone can score in the top 99.97 percentile (180) which I would have thought would be relatively uncontroversial since it's basically the same as saying not everyone can run a 10 second 100 m or 2:10 marathon.
I don't have any personal data. I can think of lots of colleagues and acquaintainces who went from litigation to solicitor, advisory, or other non-litigation roles. If it happened the other way around, it happened so early it doesn't even register--probably first year of practice.