I've got a 30" Heavy Varmint on mine, and it flexes. I may have been the post you read about, not sure....think I posted about it once before. My A5s feel slightly stiffer, when you grasp the barrel and stock between your thumb and index finger & squeeze.
Still, it doesn't affect accuracy at all. I've only shot it in a single relay of a 600 yard match, but it put up like a 199-9x. Multiple ~.75" groups at 300 yards.
My big complaint about my stock was the horrible factory McMillan paint. I hated everything about it...durability, texture, feel etc. I just had my stock custom painted, and it's lightyears better feeling.
I love McMillan stocks, just ordered another for my FTR gun this week, but I'll never order a 'painted' finish again. Molded-in finishes from here on out.