I'm not an SJ, but I will say that Si is an amazingly useful function in the world of classical music. I think that many of my colleagues are Si-doms or Si-aux.
The first man to raise a fist is the man who's run out of ideas. H.G. WELLS
The first principle is that you must not fool yourself, and you are the easiest person to fool. FEYNMAN If this is monkey pee, you're on your own.SCULLY
In some respects. I think I'm skilled with music, and I think Si provides a good pool to draw from there. And not just facts and data, of course, but "tones" of what music should sound like, and how to express a feeling through music. However, improvisation took a while to improve, and is still lagging behind other areas of skill for me.
I think Si helps in picking out "just the right note/tone/word/phrase/stroke" in art that helps us identify those things and bring them to our eyes and ears. I do it even when I'm writing sentences or talking to people, too. (unless I'm feeling lax about it)
In the 4th grade, I made an egg person out of an empty egg shell, orange yarn, some pipe cleaners for limbs and glued-on eyes. I'm afraid that was the height of my creativity. Wait, I also remember inventing a really addictive checkers type game using some marbles and some plastic candy sheet with holes in it, but that got destroyed.
Art and creativity does not always go hand in hand. Take Mona Lisa for an example. There's no denying it is clearly a piece of art. But is it creative? What's creative about it? It is a masterwork, no doubt about it, but it is still just a portrait of a woman (okay, there are numerous theories what it actually is).
I've been reading to Whack on the Side of the Head, it's a book about how can we be more creative (or actually why aren't we more creative). It has taught me that doing things efficiently and doing things creatively are the two opposing forces, and us ISTJs usually prefer the efficient way. We too can be creative, if we just remember to switch off from auto-piloting towards efficiency.
As for myself, I'd have little praticed playing piano, and I'd like to be better at it. Just haven't found the time and space for it yet (the instrument is somewhere in the back of the storage room).
Perfectionism and revisionism are probably the biggest things blocking my creativity. At times I have huge flows of ideas and concepts for stories and artwork, but little time or skill to carry them out, so I try to make note of them and carry on with normal life.