I would be fine with a guy on a special diet so long as he didn't try to force or convince me to eat the same, didn't say things to make me feel guilty for choosing to eat what I choose to eat, and didn't just talk about diet with me. My ex went vegetarian (high carb and high vegetable diet) meanwhile I started working out with a fitness professional who put me on low carb, high protein (a lot of meat and vegetables), which reallymworked well for me. But my ex kept continually trying to tell me his diet was better than mine (telling me meat gives you cancer, you can get protein from vegetables, and mocking me by calling me unfeminine for eating like a body builder even though I am far from that) ... that was very annoying!!
I absolutely would because then I wouldn't be as tempted to indulge, I'm all for sweets every no and then- all about moderation. Ontop of that, I have a pretty healthy diet so it would just work better.
Yes, that would be amazing, as I generally try to eat clean, and having a partner that was the same (mine eats horribly, but I love him anyway) would make it a lot easier and more fun, plus there'd be less temptation in the fridge.