Check out Rob Galbraith's Compact Flash performance database for the 1Ds2.
  • Extreme III 4GB is 7.181 MB/s
  • Extreme IV 4GB is 7.821 MB/s.



That's less than 10% difference. I don't think it's worth it.


The reason they are so close is because the 1Ds2 is not fast enough to take full advantage of the cards.


The 5D Mark II, on the other hand:
  • Extreme IV 4GB is 29.5 MB/s
  • Extreme III 4GB is 15.4 MB/s



So that's a camera where the difference really shows.


For myself, I think the fast cards are worth the extra expense just for reduced computer transfer times.