For a stock vehicle, do you think it is worth it? Or am I just wasting money?
I run stock powertrain for 2+ years. I tried regular, premium, and whatever is in between.
Is there a difference? Yes.
Is it worth it? It depends on you. Personally, I find the gain negligible for it to be worth the extra money.
My advice? Try it for yourself.
I say this as someone who has experience with other Hyundai engines (used both in Hyundai and Kia). You see, these engines are MADE for premium octane. However, due to the hype about gas prices, they are also configured to run on regular. I personally call this "compatibility mode". The reason I say these are made for premium, is simply by looking at their high compression ratio. I also say this based on experience.
I tried naturally aspirated Soul, and Tucson, and the premium makes a HUGE, noticeable difference both on performance and mileage. However, when I did the same on the VT, I was disappointed. Mileage was more or less the same, while performance gain was, meh. Compression ratio is also noticeably lower (9.5:1) than the aforementioned vehicles', even the NA Veloster has higher CR (11:1). I am not sure how it works, but it seems like it is the reason for increased performance, and it makes sense in my head.
In the end, I don't really call it an "increase" or a "gain". I call it normal mode. Performance is cut when not using premium, hence, "compatibility mode". Car makers are starting to make these types of engines (high compression ratio, with no engine knocks when using lower octane), but Hyundai and GM are among the first to do so.
Do a google of everything I just said, and judge if you want to believe it or not.