Streaming music despite having it downloaded offline

[Sambeez]

I have a phone plan that has a deal with spotify so none of my data used on spotify counts towards my phone bill. Therefore, I use extreme quality as it does not cost me anything. At the same time I like having my music downloaded for offline, purely for the fact that it informs me with the green and grey arrows that songs that I come across have already been added to my playlists. I have over 3,333 songs on my playlists, so I do not want to take up my whole phone's memory by downloading extreme, in fact I don't even really want to use the offline feature at all. My question is: if I have music downloaded is spotify going to use my data which is what I want, or is it automatically going to use the offline version of the song as that is saving me data? And if it is the latter is there a way of turning this off because I want to use my data so I can have the best quality.