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Just to expand on the other comments, the tag is used for styling, (http://www.w3schools.com/TAGs/tag_small.asp) So the h1 would still apply, it is just styled different to the "standard" h1 style.

Dear Devanur, Yiannis, Thanks a lot for your thorough argumentation on this. There are a few competitors that use the folders in the URL for keyword spamming purposes and I definitely don't want to pursue such a strategy. As the authority is at quite a low level, I prefer optimizing the site structure right now instead of waiting another year. Especially as most pages don't rank to well currently. Again, thanks a lot for your help guys! Best regards, Martijn

Thanks Andy! I know what to do now! Let's mark this question as 'answered!'