I am a SNG grinder (losched16 on Full Tilt). Throughout my journey I have ventured in and out of cash game play, but always found myself coming back to the SNGs. It is where I am comfortable and where I built my bankroll.
However, many of the guys playing at the high stakes games started at the SNGs then transitioned into cash and the rest is history. So there are certainly plenty of success stories about guys moving from SNG to cash. It is all about what you are comfortable with. If your ROI is in the double digits, that is great. You will have no problem continuing to build your roll.
If you transition into cash games, give it a serious shot, not just a day or two. Whatever your bankroll is, make sure you use good bankroll management (20-50 Buyins), and compare your results. If your biggest concern is simply what will provide you with the most money, then you are going to need to compare your SNG ROI with your cash game BB/100 (after at least 30K+ hands). Also look at your hourly rate. But if you want to play what you are most comfortable with, then you will have to be the judge of that after you get some cash game hands under your belt.
I hope this was helpful as I found myself asking the same questions as you are now.