I only had a small sample of the Grange 11. But it was very clear I was drinking a special wine. I still remember the countless flavours unfolding and that regal nose. It was so well made. For me, I got the same experience as drinking other Grange vintages. By no means the best I've had, but it was still Grange. And still very special.
The only noticeable difference was its lighter style, which may mean it won't be at its best at the 18 or 21 year mark (or longer). Not usually a problem for other vintages, so it may prove not to be a good choice as a birth year wine.
The value of the Grange is questionable, particularly in the 600s if it won't be in its prime in 18-21 years time. But I do fathom at the cost of producing such ludicrous wine with so much in it. I think it's pretty unique in world wine terms - and maybe why others don't copy it - it must be so expensive to produce.
With all that said, if I had the cash to buy two 11s at $375 each for a birth year excuse, then I would. I would probably open one around 2025 for another special occasion. And if it doesn't look like it has the legs to get to that 18th or 21st or whatever year you have in mind, then I'd just sell the remaining bottle on the secondary market at that point. I'd be surprised if you didn't get your money back and more if it's in tip top condition.