Ken: these cars do NOT do well over 100 km/h.
In fact, the mileage you're getting is pretty much what I would have predicted at that speed based on my own futzing around:
Source:
http://metrompg.com/posts/speed-vs-mpg.htm
Note that this was taken in warm conditions, pre-XFi cam and pre-Swift transmission. The fact you're getting 43 mpg (US) in the cold, at those speeds in a heavier, less aerodynamic (probably) convertible, tells me your mods are helping.