At the risk of pissing off the IP owners who frequent these boards (if there are any), ''tis true that IP''s are not noted for their sailing ability. These are heavy, full keeled boats that require a breeze to get moving. The newer "0" models (meaning 350, 380, 420, etc., however, are supposed to have thinner keels and to sail better than the older models, at least according to what an IP salesman told me last year at the show.
I know a fellow with an IP-32 who admitted to me once that he has never had his boat over 6 knots in speed under sail no matter what the wind conditions. I''d call that not too good.
This past summer I caught and passed a big IP (no size on its mainsail, but had to be at least 38 ft.) on a broad reach/run going from Solomons Island to Oxford, MD. Apparent windspeeds were 8-12 knots with gusts to 15 occasionally. My boat is a Pearson 27 and is a downwind rocket, but still no way I should''ve caught a boat that much larger. They gawked at me as I blew by them.
IP owners will tell you about the safety of being on the IP, the comfort of all that displacement resulting in arriving in port more rested, etc. But if you are out there 33 percent longer (pick a number) than better performing boats, I guess you need to be more rested.
There are also all kinds of stories, possibly apocryphal, of IP owners having to fire up the engine to tack, but I have never been able to verify that with an owner.
My view FWIW.