If you need to sail someplace upwind and your tacking 15 degrees lower its takes a LOT longer.
An exxcellent point. I have encountered that problem in the past as we were learning to sail the last 7 years on a Hunter 36. Even though, the AWI might show that we were bearing only 40-45 degrees off the wind, 10-15 degrees of leeway meant that our COG was 50-60 degrees off the wind. In time, we have learned to sail the Hunter closer to 30 degrees off the wind as measured by the AWI, but still in certain wind and tide conditions, the COG was well beyond that. One aspect that the broker mentioned about the IP boats is that even though they may not point as high (as measured by the AWI), they have less leeway due to the full keel. This could result in a much smaller difference in actual COG headway toward your destination. He went on to assert that is why the IPs do better in long distance races than the short "daysail" type of course generally set up by your local yacht club.
If the IP "points" 10-15 degrees worse than a typical wing keel boat (and that is a big IF as I don't have any experience with an IP), and has 5-10 degrees less leeway, then the COG difference may be minimal.
Again, it's just a matter of preference of what do you want to do with your boat. If you want to sail a triangular course for an hour or so, that requires one style of boat (and a specific level of effort on the part of the crew). If you want to take off and sail 200 miles or more across open water, that requires another. Then to create a comforatable living quarters for a month or two requires still further differences.
Different strokes for different folks.