Steve, have a search in fuel economy in technical. I have kept up a bit of a commentary over the last year.
The summary is, that I seem to have an increase of about 10%. Coming back from Canberra to Sydney a couple of weeks ago I got 16.2 Litres per 100Km ( on 91 unleaded) towing my 550 Kg camper at 110Kmh where posted. However going to Canberra into a 60Kmh Gale I only got 20L per100Km.( on E10). This is over the Great Dividing Range.
I have a long range tank so it goes a long way between fills, This makes it hard to generalise as I do all sorts of different driving. However locally my economy has increased from 22L100Km to about 20L100Km.
I am now keeping a log of each fill.
So my conclusions are
About 10% better economy
E10 gets worse economy by about 5%
Power/ responsiveness at light throttle is worse

I fitted new plugs the other day , as looking at my records I found I had forgotten , and they had done about 27K. They were all white ( beautifully lean) all over, which is different to previously.
Remember also that my car should have better economy than stock before the o2 sensors, as I have a Unichip, Thor Manifold, modded heads, high air intake, free flow muffler.
Probably the only mod that has given a noticeable difference in economy, vs power.
Regards Philip A