canegardenbay has got it right !!!!!
A cutter rig is quite difficult to sail at ''maximum'' due to the staysail and the required precision needed to get it ''pulling'', especially on a high close reach or beat. Since most boats constructed today are sloops there really isnt much hard data on how to get a cutter ''humming''.
If you take a look at the PHRF handicap ratings an Island Packet (IP38@168, IP40@156) isnt quite the ''slouch'' that many ''instant experts'' claim.
Many claim poor relatiave performance in light winds and I really dont find this to be the case on IPs or other moderate displacement cutters... especially with faired hulls, feathering props, rigid vangs to lift the boom, etc. and a good knowledge of how to properly trim the staysail.
Your statement about a lot of IP owners just dont seem to care how fast they go ..... a profound statement. So for long voyages, perhaps its the difference between "type A" or "type B" personalities; or, maybe its like sex ... longer/slower may be better than lots of ''quickies'',