Thank you for your post, however it is mistaken on several levels.
All types of petrol are blends, and mixing Hi-octane with super or regular is just another way of blending.
All grades of petrol have the same energy content of 125,000 BTU per US gallon, whereas pure ethanol has only 84,600 BTU/gal, so that E10 petrol has 120,900 BTU/gal, which is 3.28% lower so your mileage will drop at least by that amount. E10 does have a higher octane rating, but it has less energy and will give worse fuel economy.
Retarding the timing is a bad idea, whether it is done by a tuner or some other guy sitting under a tree. It is highly likely to cause other problems too. Adjusting the air/fuel maps to lean out the mixture for better mileage especially will raise combustion temperatures, increase emissions and may cause shorter engine life.
My proposed solution to blend in just enough Hi-octane to prevent detonation is the easiest, most cost-effective and scientifically robust way to deal with the OP's problem.