Entschuldigung fur meine englishe Antworte. (Sorry for replying in english). My german isnt what it used to be (right after graduating from school). Hope the english is understandable for you.

To my knowledge it is not possible to set that option in Octopus, but:

Maxfps is set per clientinstall in the /WTF/Config.wtf file. Open it in notepad or simular when wow is not running. Searach for 'maxFPS' in this file, and set the value you want. In your example the line shold read:

SET maxFPS "15"

Another way to do this is from the console ingame by sending the following command

/console maxfps 15

To set the max fps for background windows you can use command:

/console maxfpsbk 15

This is per client install. So if you have more than one wow per pc, and you want different max fps, then you will have to make 1 folder per wow client you want.