I echo what Chris has found.
I chose a “Quantum” remap, by my local, Quantum-approved garage. I received a guarantee card by Quantum. They are a very respected company and I would have no hesitation recommending them.
Mine was installed about eight years ago and has proved faultless – as Chris has experienced, I’ve not noticed a mpg improvement nor a deterioration. There probably has been a POTENTIAL improvement in mpg, but the performance now available (which makes driving the ‘chipped’ Exsis SO enjoyable) probably negates that.
Again as Chris says, the power on hills, even when heavily laden, is astonishing, and keeping up with traffic flow is just not a problem.
Here is an interesting article on ‘chipping’ engines…
https://www.comparethemarket.com/car-insurance/content/what-is-car-remapping/
And here is a link to Quantum…
https://www.quantumtuning.co.uk/
Note that there are different ways that an engine can be remapped – either by reprogramming the ecu, or by plugging in a chip, fooling the temperature sensor to just always provide a rich mixture, etc. There are some companies that I wouldn’t touch with a barge-pole, so stick with reputable companies and don’t judge the product from a “price” angle.
Good luck.
Barry