firstl your ROI is really good, just dont expect that to continue in the long term, for the most part you will be crushing low levels with an ROI of 10% for 9 handed sng. and the higher up you go, the lower your ROI will be, so dont get down when that 15% figure drops..
while this question is kind of broad, and wihtout seeing you play i cant comment too much, but if your having too many second place finishes, it generally means that your playing too tight, once you get ITM go for first, reshove lots and genearlly play very aggressively. as as mentioned above, when you get shallow stacks at big blinds, its generally correct to shove almost any hand on the button heads up.. remember this, they are getting the same crappy cards you are so by taking the initiative and shoving lots, we can in some cases force our opponant out of the match as hes folding waiting for good hands to call us or shove into us with.. so that when we do eventually get called, were playing with their money..
but like i said earlier, pretty broad question and hard to answer without seeing how you play leading into heads up, and heads up play. post some specific heads up related questions in the HA for more detailed responces..