IP, maybe this stands for Incredibly Passable?
Any of the +40 foot boats that have been mentioned in this thread will give one a pretty comfortable ride, whether it be a IP40 or a J42, you''re still going to move 15 feet vertically in 15 foot waves! Speed should be an important but not a sole deciding factor in choosing which vessel best fits your needs. The speed difference of a minute-a-mile over long passages could be the difference in a squall between taking a lot of sailcloth down offshore or removing the cockpit awning in the harbor.
There is no doubt that the IPs are robustly designed and built. In the trade winds you could probably not find a better suited boat however the way 95% of us use or boats, swift is better than slow. My money is on the 402!