Ok getting this out of the way. The CVT would not get better mileage than the 5 speed no matter what. There is always going to be less drag and weight with a manual transmission; however, there is room for improvment with lean burn. Looking at
http://www.honda.co.jp translated through babelfish translator the fuel efficiency numbers for the japanese Insight are 36 Km/L for the manual and 32 Km/L for the automatic.
Here's the clincher though. The manual Insight in the US is a 10.8 to 1 compression ratio where the automatic is a 10.3 to 1. The Japanese page lists 10.8 to 1 for both transmissions so we're talking a different engine here. I was going to say perhaps you could try and get the japanese ECU, with no guarantee's it would work, but this puts a big dampener on the possibility.
Anyway back to those fuel economy numbers. Comparing 36 to 32 is only about a 9% difference, whereas you compare 56mpg to 64mpg yes thats a larger difference at 13% (granted this is the japanese fuel economy test vs the US's fuel economy test). So if you figure this in you theoretically are looking at more like 58 combined mpg with the CVT with lean burn.
I'm guessing they had some incentives to make SULEV car's here. I'm also guessing at some point they may have considered not making our 5 speeds without lean burn so they could all be SULEV, but luckily they didn't. I'm not saying the ECU swap wouldn't work, but the fact that it's a different engine makes me lean the other way. Plus really, if ECU's cost anything like what they do here it would probably be really expensive to get one shipped here.