The waitress was skeptical when I asked for extra hot too.Sriracha isn't all that hot. But my tolerance for chilies is ridiculously high.
If you really want to step it up a bit order something double Thai hot. About a month ago I ordered a curry dish at a place that rated Thai hot as 4 stars, I got 8 stars and the waitress brought out some fish sauce loaded with chilies, mmmm.
It usually takes a few visits to a new place for me to convince them that it's ok to light this white boy up.