Whisk together mayonnaise and sriracha. Mix kara-áge and cayenne pepper. Toss 1 cup shrimp per serving in kara-áge mixture and deep-fry until golden and crisp. Spread 1/4 cup Sriracha mayonnaise on cup sides of 1 sub roll and fill roll with 3 tomato slices, 1/2 cup lettuce and deep-fried shrimp.