The difference between E10 and E5 is less than 2%
My own experience of premium fuels in a Euro4 1.2 Panda 169 is much better than that, but it's a complex subject and that doen't mean what works for me will work out well for everyone.
When the change to E10 came, I made the decision to switch to Esso Synergy Supreme+ (putatively an E0 fuel in many, but not all, parts of the UK) to see if I could detect any change from the supermarket E5 it had been running on for most of its life. This decision is constantly under review, but I've enough real world data to know that, for me, it's worth the extra cost, both in terms of economy and driveability.
I was surprised to see a pretty much instant improvement in both fuel consumption and bottom end torque. A quick check on the newly-reset trip computer showed a noticeable gain, and pickup from 25mph in 5th under a light throttle was definitely stronger. All this was obvious less than three miles after refuelling an intentionally near-empty tank.
I have a regular motorway journey of 100+ miles, and always start the journey from cold with the trip reset, which gives me a useful ongoing check on economy. (Incidentally, this was the first indication I had of a failing thermostat, when I saw an unexpected 10mpg drop). At a steady 55mph, on supermarket E5, I'd be looking at somewhere between 73-75mpg on the trip for a fine daylight journey. Only once, with exceptional care and highly favourable weather, I magaged 80.1mpg. Since switching to premium fuel, I've never seen below 80mpg on the trip in summer, and generally the figure is in the 82-84mpg range.
Real world figures are of course nowhere near that, but Fuelly does a running average over the last ten tankfuls, and that's moved from just over 55mpg on supermarket E5 to just under 60mpg on Esso Supreme+. If I get a minute, I'll pull up the figures and do a comparison between the last 12 months on supermarket fuel, and the first 12 months on Supreme+.
I'd say the difference is somewhere in the range of 5%-10%; enough to pay for the increased cost of the fuel. As the subjective driveability is noticeably better, I see no reason to go back.
So, what's going on?
I'd always thought that premium fuels would mainly benefit high compression engines being driven hard, but that isn't exactly true. If the ECU can adjust the ignition timing on the fly (and it seems the 1.2 FIRE can do this), then a higher octane fuel means the engine can run more advanced without knocking, and this improves both torque and economy. The big surprise for me was how much better the engine performed when pulling under load on a light throttle. I suspect most of the economy gains I've seen result not from the increased energy in an E0 fuel (the theoretical difference isn't that much), but from the engine's ability to run more advanced under load for any given throttle setting.
Your mileage may vary (quite literally!), but if you're an ecodriver, it might just be worth giving a premium fuel a fair trial. OTOH, if your driving style is to regularly use more than 2000rpm in the intermediate gears, you might not see much difference.