There are a few car companies that profess great fuel economy and performance in the same breath, but they don't tell the whole truth. You need a small displacement engine and a light car to have a chance at good fuel economy. The small Lotus is one that works, but it is only a 2 seater.
The modern engines use computers and a mirade of sensors to regulate timing, the human intervention has been taken out of the formula. There is no way to modify, manually, the timing on 850 engines. A hardware programmer might be able to do it, but that requires a lot of computer experiance not normally found in a DIY garage.
The 850 normally gets around 20-21mpg in city traffic and 27-30mpg on the highway. Not bad for a "touring" car built for 4 adults that weighs in over 3,000 lbs. Compare that mileage to 2008-2009 cars that have the same 0-60 times and comfort level.
You refer to using cheaper octane levels with the NA 850. That model has a compression ratio of 10:1, not suitable for 87 octane. The only way to "fix" that would be to put in shorter connecting rods to lower the compression ratio and that would result in a substantial decrease in bhp. That is why there are turbos, lower compression when 167bhp is not needed and higher bhp when it is.

Klaus
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Just driving a 1998 V70R :)