As far as pushing the ST104 to the limits I have an interesting story for you.
Last year, when I still had my Carve 121 (currently replaced with Futura122) I tried an experiment which was not completely intentional.
I let my friend to take my Carve with 6.5 while I was standing on the shore and watching.
I had also my 8m Retro rigged and my ST104 with True Aims 36cm weed fin sitting on the shore and I was thinking that I wish I had something decent to sail myself. Then an idea came to my head, why not to try my 8.0 on ST104. But from my experience on the Carve, for this sail I needed 44cm fin otherwise I was spinning the tail on 36cm. Than I thought, what the heck, I have nothing to lose and I tried this setup.
Surprisingly, I had no any problem with spinning and was comfortably zipping back and forth enjoying jibes. I tried it many more times and always loved it but since I got my Futura I want to use my 8.0 on on this board.