^^^ It isn't always better to use higher octane. There was a post a while back about GT owners using premo gas and feeling less power, and quite a few people agreed that some of the gas might not have been burning off. I can see that as entirely possible if you're using something like 100 octane on a stock GTS.
On a GT is a totally different story as its running lower compression. So 87 is good enough. But on a GTS with higher compression, the higher octane is necessary plus I really think it thrives on it, but only up to a certain point...like 94 tops, 100 is overkill.