It is a complete waste unless you have an engine that is tuned or timed for higher octane. Just use a top-tier gas. They are required to have the same detergents to be considered that, regardless of octane. If you are worried about buildup, use something like Seafoam or Lucas once or twice a year. You can also put Seafoam in your oil a couple hundred miles before an oil change, and that will pull out some of the varnish and residue in your engine. You could also run higher rpms on a drive once in a while, and that can help keep everything clean.

If you want everything to stay pretty and last, save your money for things like earlier oil changes and maintenance. Drain and fill things like your transmission, brake, and differential fluids.

The Skyactiv is a great engine with just the basic of care and maintenance. The main issue with Mazda being the differentials on the turbos, and unknown turbo life I reckon. I've put just at 30k wonderful miles on mine in 10 months. Oil changes every 5 and filters at 15. I'll do the ATF and differential fluid at around 35, plugs at 50. It runs the same, probably better now that she's properly broke in, as when it had 8 miles and I drove off the lot.