Preheat the oven to 375 degrees F. Combine the allspice, oregano and garlic powder in a bowl. Pierce the pork all over with a fork. Season with 1/2 teaspoon salt and a few grinds of pepper; rub with the spice mixture.
Heat 1 tablespoon olive oil in a large ovenproof nonstick skillet over medium-high heat. Add the pork and cook, turning, until browned all over, about 5 minutes. Transfer the skillet to the oven. Cook until a thermometer inserted into the center of the pork registers 145 degrees F, about 20 minutes. Transfer to a cutting board and let rest 5 minutes.
Meanwhile, heat 1 tablespoon olive oil in a medium saucepan over medium-high heat. Add the couscous and cook, stirring, until toasted, 5 minutes. Add 1 3/4 cups water, 1/2 teaspoon salt and a few grinds of pepper. Bring to a simmer, then reduce the heat to low; cover and cook until the liquid is absorbed, about 15 minutes.
Fluff the couscous with a fork; let cool. Add the feta, cucumbers, herbs, sun-dried tomatoes, lemon juice and the remaining 1 tablespoon olive oil; toss. Season with salt and pepper. Serve with the pork and lemon wedges.