As a person who (at a glance) fits all the "can't handle spice" stereotypes, yet loves spicy food, I often find myself in the awkward position of having to ask for corrections after my food comes out. Some restaurants don't have chili powder on hand for the table, and I have to send the dish back, prolonging my meal. Even if I order a dish as spicy as the restaurant offers it (5/5, "extra hot", "Thai spicy", whatever), my dinner partners are almost always served spicier food than I am, even if they order their food lower on the scale.
For example, when joining my Asian friends (I am not Asian) for a meal at a Thai restaurant, even if we all order the same dish, at the same level of spiciness, it's a total crapshoot whether my meal will come with any spice whatsoever. We've all noticed this as an issue, and tried each other's food, so I'm sure I don't have some abnormal resistance to spice.
What is a polite way to impress upon my servers that I know what I'm asking for? I've had mixed success with saying things like
"5/5 - and I really mean it!"
"really, just serve me a plate of lava, I'll be fine"