Place pork on a grill rack coated with cooking spray; grill 6 minutes on each side or until a thermometer inserted into thickest portion of pork registers 14
Remove from grill; let pork stand 5 minutes. Slice pork crosswise in half; slice each half, lengthwise, into thin strips. Cool.
Combine juice and next 6 ingredients (through serrano) in a large bowl.
Add pork to juice mixture; toss to coat.
Add mint and next 3 ingredients (through cucumber) to bowl; toss. Arrange 2 cups lettuce on each of 4 plates; top each serving with about 2/3 cup pork mixture.