You have a habit of altering arguments and making assumptions. I didn't say anything about RAW vs Jpeg.
If JPEG is your file format of choice, then you will experience 0 issues with a 400x card.
You are talking about the speed difference of a honda civic vs a formula one car, both @ highway speeds.
In otherwords, you talk about these $500+ 1000x cards, when your file format is jpeg. It's an irrelevant argument and 1000x becomes unnecessary and overkill.
I shoot in ALL raw, and have never hit the buffer. Even in RAW, the D800 shoots at 4fps. That means, that the 1000x speed gets you an extra 3 or 4 frames over the 400x.
People will say "Oh those extra 3 frames can make the difference of getting the shot and not getting the shot". For those people, the D800 and 4fps is not for you.
If you need to motor drive for sports, then you need more than 4fps. Not to mention, when you do hit the buffer, it's about 30-40 seconds with 1000x.
Theoretically, you get an extra frame every 2 seconds in compressed raw and 2 extra frames total until you hit the buffer.