Portuguese food is not built around chilli, so when I want real heat I choose the kitchen for it instead of hoping the piri-piri bottle will save the meal. In Porto, these are my useful starting points.
For curry: Thali or Turmeric
Thali at Rua da Fábrica 34 has lamb madras, chicken curry and palak paneer. The madras is the obvious place to start if you want some warmth; ask how hot that batch is before ordering. Map
Turmeric on Rua Formosa is Pakistani and Indian, with chicken kadhai and garlic naan among the dishes on its menu. Ask for the heat level you want rather than assuming a familiar curry name means the same thing as it does at home. It is halal and alcohol-free. Map
For Thai or piri-piri
Thailander on Rua das Oliveiras is my central Thai option. Tell the server how much chilli you usually like and ask what they recommend from that day’s menu. If you’re eating grilled chicken elsewhere, ask “Tem molho piri-piri?” (“Do you have piri-piri sauce?”) and request it on the side. Table sauce is easier to control than a dish whose base is already hot.
One useful phrase: “É picante? Pode trazer o molho à parte?” (“Is it spicy? Could you bring the sauce separately?”) If you have a serious allergy, explain that separately; chilli heat and allergy information are not the same conversation.
I’d rather order a dish that tastes good at its proper heat than challenge a kitchen to make everything scorching. Ask plainly, take the staff’s answer seriously, and keep piri-piri within reach if you want to turn it up.