Think of a sunflower seed being the equivalent of a large bagel for you or me w/ portion size. Sunflower seeds are extremely high in fat so should be fed in moderation ideally. It's all about the common sense. If your rat is skinny, more is okay, if your rat is chunky, then less.
As for bananas and potato, be cautious with both and make sure they aren't green. Raw sweet potato, and green potato skins and eyes contain a toxin. A friend of mine lost her heart rat when she fed her raw sweet potato. As for green bananas, they inhibit the absorption of starch.
What I do myself, rather than feeding my rats 'extra' treats, is /hand/ feed them their regular lab blocks and regular fruit and veggie diet as treats. They have some portions of it they like more, so those I reserve for my 'spoil' sessions. For example, I always feed them their green peas when they are out of the cage as a reward for coming when called, or I'll put it in a big shallow dish filled with aquarium stones water so they can play the 'fishing for peas' game.
Here's a link to a healthy rat diet which contains a list of safe and not so safe foods.
Yes, I always remember when I was doing my research before adopting ratties, that in the nutrition do's and dont's, I always repeatedly read that raw potato was a no-no (although I think cooked potato was okay?) and bananas should not be fed when they are still green or unripe. So since my boys LOVED banana, I always waited until the bananas started to brown and got softer, then fed them slices of it. And yes, a rat veteran on the forum told me early on, "Think of sunflower seeds as the equivalent of humans eating Ding-Dongs and Doritos..ratty junk food and give these as treats very sparingly. Lots of fat!