Well, you would be surprised at how many people have a layer of plaque in some of their arteries and 15% is very small. It could be the result of your high blood pressure and accelerated by smoking. Due to family history of heart disease, have you had a cholesterol blood test? this is the next step. I think personally, if you get your cholesterol checked, keep your blood pressure under control, make life style changes such as regular exercise and a healthy diet, then there is no reason why your carotid arteries should get any worse.
There is a chance that you may have a small amount of disease in your coronary arteries, but then again, everyone has that chance.I think you would certainly know about it if the levels of disease were significant and making lifestyle changes will have a huge impact. One thing to remember, one of the risk factors is stress. To worry about these things continually will cause stress and make your disease worse, so although difficult, it's best to just come to terms with the fact you have a small amount of disease and move forward, knowing you can nip it in the bud now before it gets worse.