Thank you everybody for your great feedback and thoughts!
The images from the 300 2.8 were totally amazing - seeing as I already have the 70-200 2.8 that might also be an option which would give me more flexibility (+1.4xTC or 2xTC). Most of the time I have enough room to stand far away.
I also agree that I should spend more time learning from the pros than to invest in gear - but the lenses are just sooooo tempting and I am extremely affected by G.A.S.
I think photography requires continuous improvement in both aspects, technique and gear. As you move on with your technique, better tools can help you to fully utilize your newly learned skills.
I often am around fashion shows or shoot weddings for friends who can't afford a 'real' photographer or who want a 2nd shooter. Here are some examples, critique is very welcome as I know I still have a lot to learn. At the moment I am trying to improve the 'posing people' part.
The motorbike picture was taken using my Fuji X100S, the other ones using my 5D3.