I think it might be wrong to think Gitana is much faster than IDEC in all conditions. From what we've seen, it seems Gitana is indeed much faster in medium conditions, because they can get foiling when IDEC couldn't, but i think this gap is reduced in heavy conditions, where foiling becomes complicated and which IDEC with its small mast was optimised for. I still think gitana is a faster boat overall, but that is more due to their excellent speeds in transitional zones with foiling conditions (allowing them to get to another system without slowing down to much which is a big advantage!) than their top high speed averages. We havent seen a boat break the 24 h record in 10 years but i hope Gitana is the one that does it.
IDEC's run in the Indian cannot be understated, 6 days at more than 35 knots in front of the low is amazing. It is also hard to beat because you have to stay in the front to have those speeds. It has to have the right speed and travel with you. Maybe some day we'll see Gitana, Sodebo or one of the incoming Ultims sailing several days at 40 knts in the right front but you do have to get really lucky.
Gitana in the same low that Idec had a few years ago probably wouldn't have been faster on that amazing run because IDEC never fell behind the front, they just slowed down and gybed when it fizzled out. I remember them saying that it was a lot of work staying in front of the low, and that they almost fell behind at one point, so Gitana might have had an easier time staying in front, but hardly could have gone faster in the same low because the farther forward you are, the less wind there is.
Some day we will get boats able to punch in front through the next system, but we haven't seen Gitana do that (yet )