Whisk together BBQ sauce, grape jelly, Sriracha sauce, Worcestershire sauce, garlic and pepper. Pour half of BBQ sauce mixture over pork tenderloin. Remove excess air from zip top bag and seal. Place pork tenderloins in refrigerator for an hour to overnight to marinate. Cover remaining marinade/ sauce with plastic wrap and store in the refrigerator for later use.
Bake or grill pork tenderloins until they reach an internal temperature of 160 degrees F. Remove from oven or grill and wrap with aluminum foil for five to ten minutes to rest.
When ready to serve, brush with additional BBQ sauce. Serve with additional sauce on the side.