I think I'd leave it, in case you want to sell it later. 15mm poking out of the top of the stem isn't really doing any harm. One of my caps and matching spacers would look nicer, obvs.
My thinking too regarding the steerer, although slammed would look great. Already have a one of your caps in my spares box, might get it on when I sort the bung issue out as a finishing touch.
I think I'd leave it, in case you want to sell it later. 15mm poking out of the top of the stem isn't really doing any harm. One of my caps and matching spacers would look nicer, obvs.