Basically, if the owner's manual of the car recommends a certain fuel, that's the fuel it's designed to work best with. If your car requires premium fuel, then there is a chance for something more severe than simple a couple less horsepower or lower mileage. However, if it onle "recommends" premium fuel, than your engine can work fine with all of them, you just sacrifice a few horsepower (usually no more than 5-10) and a couple mpg's.
If your engine isn't designed to exploit the higher-octane fuels, it shouldn't be much (if any) worse, you'd simply be throwing money down the toilet.
It would be like putting 1 small heirloom tomato in a big pot of tomato sauce with a bunch of regular tomatoes. Unless the receipe is built to showcase the "fancier" tomato, you'd never know it was there, and are just wasting your money. However, if you're making a nice caprese salad and the recipe can support the extra complexities the heirloom tomato can offer, well then the upgrade would be worth it.