The company seems to specialize in ECU tuning and as such, may be able to give you more MPG or performance than a stock car. However there is a trade off, more performance could n=mean less economy, more economy means less performance. The stock configuration is a blend between both worlds. not only is it pricey, ($237) but you would have to get your car over to London, and even then the tuning may not work the same once you got back to the USA.
There are more local places that will do the tuning and I doubt if it is cheap. up to you.
Evilution is hosted out of England where there are a lot more different models of the brand.
Plus mpg in the UK is different than mpg in the USA. That car would be something like 50 mpg here. The European models are typically well known to have better fuel economy anyway. They gave us the "middle of the road" model whilst Europeans enjoy some variation in their smart trims.
I have heard that the octane of the gas is what matters, and I believe it. The better the octane of the gasoline you use, the better the milage will be. It's something to do with the molecular structure of the carbon chain in the fuel. The longer the carbon chain, the better the milage.
UK and Canadian gallons are measured in Imperial gallons which are larger than US gallons therefore we in Canada and those in the UK will always get 'better' MPG.
Running premium, theoretically you will also get slightly better mpg depending on how you drive as the fuel is more efficiently combusted, technically however with other factors thrown in it may or may not occur.
However if your question is actually more along the lines of wanting to justify using regular instead of premium I would think twice about it. I can tell you that if you let you car sit for extended periods of time running regular which has higher concentrations of ETHANOL will cause problems with your vehicle especially older cars that did not have their fuel systems designed to deal with the corrosive nature of ethanol in fuel. Therefore if you only drive your smart during the summer or worse if you only drive a car maybe once a year and do not run through the entire tank of gas you could damage your fuel system.
How do I know this???? Well my 86 T-Bird TC just came back from the shop, I only drive this car once or twice a year and as a result of letting it sit with regular fuel in the tank the ethanol mix became corrosive and ate away at my fuel pump, and fuel regulator to the point that they failed. When we took out the parts which were about 3 years old the fuel pump had large quantities of yellow sludge coating the filter and pump. The regulator was also damaged and required replacement. Since fuel pumps live in the gas tank on most new cars this is not a cheap repair if you have to drop the tank in order to fix the problem.
Here is a link to an article on what ethanol can do if left long term in a car. E15 and Engines - Can Ethanol Damage my Engine If you drive the car regularly and run through tanks of gas quickly I guess regular won't be the end of the world, however if you do not run through a tank of gas at least every few months you could cause damage depending on how our smarts fuel systems have been designed to deal with ethanol.
Our smarts are made in France however I think that any car sitting with ethanol fuel in the tank for long periods of time may be at risk even when used with fuel stabilizer which I question it's effectiveness. For example the shop that services our snow blower, lawn mower and generator had me switch over to premium fuel that did not contain ethanol as it caused major issues with the carb in my Honda generator.
And to make things more complex not all premium gas is ethanol free from what I have discovered. This site tracks brands whose premium is supposed to be free of ethanol, however looking at the list some of the entries are pretty outdated....