I have been told by many people that my experience in the tunnel should help me immensely when I start AFF. As a matter of fact, I have scheduled my AFF, and told my coach I'd like to work on the elements I will need for it. He told me that I'd already gone beyond what I'd need to pass AFF.
I did feel that I understood the arch much better during my tandems because I'd worked on it in the tunnel. I had a difficult time getting stable in the tunnel and my arch was terrible. I've been able to focus on that and really improve it, and I think it's given me more confidence that I will be able to get stable during solo freefall.
Still, why not do both? (Some before, some during?)