I have a ST115 and always used it down till 4.7 with no problems at all. Volcans yes, spocks no but that?s not the board?s fault
I am a little heavier though (100kg) but 5.4 will for sure be ok for your weight. With my weight at 4.7 there are already some guys on their 4.0 s on their move. So I think as long as you can handle 5.4 you can also handle the ST115. I have to say that I am not using the stock fin though. With your weight I would take something around 28-30 cm more freeride/freemove biased. I do so too and in my opinion the ST is not that much slower because of a bit swept back fin.