We steam our veg Dave. But do boil/bake our spuds. I'm not a regular with sweet potatoes so don't know the best way.
Steaming usually holds more of the goodness in and a general rule of them is that overcooking will loose some of the nutrients but I believe this is more of a statement on the vitamins and minerals in them rather than any loss in carb/protein content, unless of course it becomes charcoal
We actually have stirfrys quite often but without a lot of the fatty oils. Peppers, onions, garlic, mangetout etc. plus the steamed veggies(brocolli and cauli) on the side. Occassionally we use straight to wok noddles. Yum Yum