Sorry to interrupt the ethanol hate fest, but just going back to Laker's original dilemma of a bit of a thirsty GRiSO, my recent experiences with the Arrow midpipe may be helpful.
The pretzel is absolutely necessary for low down grunt. The less bendy midpipes from Arrow and Agostini (when mapped properly) basically mean a lot more top end power, but somewhat pedestrian low and mid go comparatively. If his Rexxer map was done right, Laker will know his GRiSO goes like a dog shot up the arse once the revs get north of 6000 RPM.
Now, my experience with Rexxer (and other tuners) is that they only modify the main fuel map, leaving the delta-fuel and other tables alone. This is a trap for the unwary, as without the pretzel, the factory delta-fuel will fuel the right cylinder all wrong like. Not to mention the the other correction tables doing your fuelling a naughty.
Sorry about the long-winded preface, but Laker, I reckon your lower mpg is due to the delta-fuel slobbering fuel at certain throttle positions.
I would like to help, and I reckon there is a couple of ways to go. You can either send me your Rexxer map and I'll tweak it, or I can send you my Arrow-Mistral map and you can try it. Worse case scenario, if it don't work, you can stick your original Rexxer map back in.
What you reckon?