hmm ok outa my knowledge area here but the phrase 'like to stall 1st time after start' is interesting.... that might indicate a issue with fuel supply (or i guess possibly the cold start injector)...

Now the fuel economy thing is interesting but what i find really interesting is the assertion about power...of course a comparative dyno reading owuld be more objective..but lets say your right...

what im wondering here is the health of the O2 sensor..my thinking is like this...

the o2 sensor adjusts the mix, but if its on the shady side you might be looking at a mix more less influenced by the 02 sensor mor by the default settings...

if the default settings were designed for a lower octane fuel the higher octane fuel would essentially make the engine run rich and therefore not be very fuel efficient and down on power...

thus the lower octane fuel would more closely match the default mapping and therfore produce better fuel effieciency and power...

now I cant claim to 'know' this for sure but the reasoning sure makes sense to me... I would ty measuring the O2 sensor ...