I don't think the outfits have anything to do with levels of power. Though, it might be probable that one may assume that because of the fact that the main character (Bloom) has a rather complex enchantix outfit along with a large amount of power, but that's because she's the main character (the creators had to make her extra-special for people to "like her the best"...)
I don't see Flora as one being one of the more powerful members of the Winx Club... nature is precious, so I don't see how it could be a strong fighting force when it comes to battling, but that's just me diving into the subject too deep.